[{"id":3680671,"web_url":"http://patchwork.ozlabs.org/comment/3680671/","msgid":"<20260422092327.3f629ad6@shazbot.org>","list_archive_url":null,"date":"2026-04-22T15:23:27","subject":"Re: [PATCH v1 1/2] vfio: add callback to get tph info for dma-buf","submitter":{"id":91887,"url":"http://patchwork.ozlabs.org/api/people/91887/","name":"Alex Williamson","email":"alex@shazbot.org"},"content":"On Mon, 20 Apr 2026 11:39:15 -0700\nZhiping Zhang wrote:\n\n> Add a dma-buf callback that returns raw TPH metadata from the exporter\n> so peer devices can reuse the steering tag and processing hint\n> associated with a VFIO-exported buffer.\n> \n> Keep the existing VFIO_DEVICE_FEATURE_DMA_BUF uAPI layout intact by\n> using a flag plus one extra trailing entries[] object for the optional\n> TPH metadata. Rename the uAPI field dma_ranges to entries. The\n> nr_ranges field remains the DMA range count; when VFIO_DMABUF_FLAG_TPH\n> is set the kernel reads one extra entry beyond nr_ranges for the TPH\n> metadata.\n> \n> Add an st_width parameter to get_tph() so the exporter can reject\n> steering tags that exceed the consumer's supported width (8 vs 16 bit).\n> When no TPH metadata was supplied, make get_tph() return -EOPNOTSUPP.\n> \n> Signed-off-by: Zhiping Zhang \n> ---\n> drivers/vfio/pci/vfio_pci_dmabuf.c | 62 +++++++++++++++++++++++-------\n> include/linux/dma-buf.h | 17 ++++++++\n> include/uapi/linux/vfio.h | 28 ++++++++++++--\n> 3 files changed, 89 insertions(+), 18 deletions(-)\n> \n> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c\n> index b1d658b8f7b5..fdc05f9ab3ae 100644\n> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c\n> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c\n> @@ -17,6 +17,9 @@ struct vfio_pci_dma_buf {\n> \tstruct phys_vec *phys_vec;\n> \tstruct p2pdma_provider *provider;\n> \tu32 nr_ranges;\n> +\tu16 steering_tag;\n> +\tu8 ph;\n> +\tu8 tph_present : 1;\n> \tu8 revoked : 1;\n> };\n> \n> @@ -60,6 +63,22 @@ vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,\n> \t\t\t\t priv->size, dir);\n> }\n> \n> +static int vfio_pci_dma_buf_get_tph(struct dma_buf *dmabuf, u16 *steering_tag,\n> +\t\t\t\t u8 *ph, u8 st_width)\n> +{\n> +\tstruct vfio_pci_dma_buf *priv = dmabuf->priv;\n> +\n> +\tif (!priv->tph_present)\n> +\t\treturn -EOPNOTSUPP;\n> +\n> +\tif (st_width < 16 && priv->steering_tag > ((1U << st_width) - 1))\n> +\t\treturn -EINVAL;\n> +\n> +\t*steering_tag = priv->steering_tag;\n> +\t*ph = priv->ph;\n> +\treturn 0;\n> +}\n> +\n> static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,\n> \t\t\t\t struct sg_table *sgt,\n> \t\t\t\t enum dma_data_direction dir)\n> @@ -89,6 +108,7 @@ static const struct dma_buf_ops vfio_pci_dmabuf_ops = {\n> \t.pin = vfio_pci_dma_buf_pin,\n> \t.unpin = vfio_pci_dma_buf_unpin,\n> \t.attach = vfio_pci_dma_buf_attach,\n> +\t.get_tph = vfio_pci_dma_buf_get_tph,\n> \t.map_dma_buf = vfio_pci_dma_buf_map,\n> \t.unmap_dma_buf = vfio_pci_dma_buf_unmap,\n> \t.release = vfio_pci_dma_buf_release,\n> @@ -211,7 +231,9 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,\n> \t\t\t\t size_t argsz)\n> {\n> \tstruct vfio_device_feature_dma_buf get_dma_buf = {};\n> -\tstruct vfio_region_dma_range *dma_ranges;\n> +\tbool tph_supplied;\n> +\tu32 tph_index;\n> +\tstruct vfio_region_dma_range *entries;\n> \tDEFINE_DMA_BUF_EXPORT_INFO(exp_info);\n> \tstruct vfio_pci_dma_buf *priv;\n> \tsize_t length;\n> @@ -228,7 +250,10 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,\n> \tif (copy_from_user(&get_dma_buf, arg, sizeof(get_dma_buf)))\n> \t\treturn -EFAULT;\n> \n> -\tif (!get_dma_buf.nr_ranges || get_dma_buf.flags)\n> +\ttph_supplied = !!(get_dma_buf.flags & VFIO_DMABUF_FLAG_TPH);\n> +\ttph_index = get_dma_buf.nr_ranges;\n> +\tif (!get_dma_buf.nr_ranges ||\n> +\t (get_dma_buf.flags & ~VFIO_DMABUF_FLAG_TPH))\n> \t\treturn -EINVAL;\n> \n> \t/*\n> @@ -237,19 +262,21 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,\n> \tif (get_dma_buf.region_index >= VFIO_PCI_ROM_REGION_INDEX)\n> \t\treturn -ENODEV;\n> \n> -\tdma_ranges = memdup_array_user(&arg->dma_ranges, get_dma_buf.nr_ranges,\n> -\t\t\t\t sizeof(*dma_ranges));\n> -\tif (IS_ERR(dma_ranges))\n> -\t\treturn PTR_ERR(dma_ranges);\n> +\tentries = memdup_array_user(&arg->entries,\n> +\t\t\t\t get_dma_buf.nr_ranges +\n> +\t\t\t\t\t(tph_supplied ? 1 : 0),\n> +\t\t\t\t sizeof(*entries));\n> +\tif (IS_ERR(entries))\n> +\t\treturn PTR_ERR(entries);\n> \n> -\tret = validate_dmabuf_input(&get_dma_buf, dma_ranges, &length);\n> +\tret = validate_dmabuf_input(&get_dma_buf, entries, &length);\n> \tif (ret)\n> -\t\tgoto err_free_ranges;\n> +\t\tgoto err_free_entries;\n> \n> \tpriv = kzalloc_obj(*priv);\n> \tif (!priv) {\n> \t\tret = -ENOMEM;\n> -\t\tgoto err_free_ranges;\n> +\t\tgoto err_free_entries;\n> \t}\n> \tpriv->phys_vec = kzalloc_objs(*priv->phys_vec, get_dma_buf.nr_ranges);\n> \tif (!priv->phys_vec) {\n> @@ -260,15 +287,22 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,\n> \tpriv->vdev = vdev;\n> \tpriv->nr_ranges = get_dma_buf.nr_ranges;\n> \tpriv->size = length;\n> +\n> +\tif (tph_supplied) {\n> +\t\tpriv->steering_tag = entries[tph_index].tph.steering_tag;\n> +\t\tpriv->ph = entries[tph_index].tph.ph;\n> +\t\tpriv->tph_present = 1;\n> +\t}\n> +\n> \tret = vdev->pci_ops->get_dmabuf_phys(vdev, &priv->provider,\n> \t\t\t\t\t get_dma_buf.region_index,\n> -\t\t\t\t\t priv->phys_vec, dma_ranges,\n> +\t\t\t\t\t priv->phys_vec, entries,\n> \t\t\t\t\t priv->nr_ranges);\n> \tif (ret)\n> \t\tgoto err_free_phys;\n> \n> -\tkfree(dma_ranges);\n> -\tdma_ranges = NULL;\n> +\tkfree(entries);\n> +\tentries = NULL;\n> \n> \tif (!vfio_device_try_get_registration(&vdev->vdev)) {\n> \t\tret = -ENODEV;\n> @@ -311,8 +345,8 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,\n> \tkfree(priv->phys_vec);\n> err_free_priv:\n> \tkfree(priv);\n> -err_free_ranges:\n> -\tkfree(dma_ranges);\n> +err_free_entries:\n> +\tkfree(entries);\n> \treturn ret;\n> }\n> \n> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h\n> index 133b9e637b55..b0a79ccbe100 100644\n> --- a/include/linux/dma-buf.h\n> +++ b/include/linux/dma-buf.h\n> @@ -113,6 +113,23 @@ struct dma_buf_ops {\n> \t */\n> \tvoid (*unpin)(struct dma_buf_attachment *attach);\n> \n> +\t/**\n> +\t * @get_tph:\n> +\t * @dmabuf: DMA buffer for which to retrieve TPH metadata\n> +\t * @steering_tag: Returns the raw TPH steering tag\n> +\t * @ph: Returns the TPH processing hint\n> +\t * @st_width: Consumer's supported steering tag width in bits (8 or 16)\n> +\t *\n> +\t * Return the TPH (TLP Processing Hints) metadata associated with this\n> +\t * DMA buffer. Exporters that do not provide TPH metadata should return\n> +\t * -EOPNOTSUPP. If the steering tag exceeds @st_width bits, return\n> +\t * -EINVAL.\n> +\t *\n> +\t * This callback is optional.\n> +\t */\n> +\tint (*get_tph)(struct dma_buf *dmabuf, u16 *steering_tag, u8 *ph,\n> +\t\t u8 st_width);\n> +\n> \t/**\n> \t * @map_dma_buf:\n> \t *\n> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h\n> index bb7b89330d35..a0bd24623c52 100644\n> --- a/include/uapi/linux/vfio.h\n> +++ b/include/uapi/linux/vfio.h\n> @@ -1490,16 +1490,36 @@ struct vfio_device_feature_bus_master {\n> * open_flags are the typical flags passed to open(2), eg O_RDWR, O_CLOEXEC,\n> * etc. offset/length specify a slice of the region to create the dmabuf from.\n> * nr_ranges is the total number of (P2P DMA) ranges that comprise the dmabuf.\n> + * When VFIO_DMABUF_FLAG_TPH is set, entries[] contains one extra trailing\n> + * object after the nr_ranges DMA ranges carrying the TPH steering tag and\n> + * processing hint.\n\nI really don't think we want to design an API where entries is\nimplicitly one-off from what's actually there. This feeds back into\nthe below removal of the __counted by attribute, which is a red flag\nthat this is the wrong approach.\n\nIn general though, I'm really hoping that someone interested in\nenabling TPH as an interface through vfio actually decides to take\nresource targeting and revocation seriously. There's no validation of\nthe steering tag here relative to what the user has access to and no\nmechanism to revoke those tags if access changes. In fact, there's not\neven a proposed mechanism allowing the user to derive valid steering\ntags. Does the user implicitly know the value and the kernel just\nallows it because... yolo? Thanks,\n\nAlex\n\n> *\n> - * flags should be 0.\n> + * flags should be 0 or VFIO_DMABUF_FLAG_TPH.\n> *\n> * Return: The fd number on success, -1 and errno is set on failure.\n> */\n> #define VFIO_DEVICE_FEATURE_DMA_BUF 11\n> \n> +enum vfio_device_feature_dma_buf_flags {\n> +\tVFIO_DMABUF_FLAG_TPH = 1 << 0,\n> +};\n> +\n> +struct vfio_region_dma_tph {\n> +\t__u16 steering_tag;\n> +\t__u8 ph;\n> +\t__u8 reserved;\n> +\t__u32 reserved2;\n> +};\n> +\n> struct vfio_region_dma_range {\n> -\t__u64 offset;\n> -\t__u64 length;\n> +\tunion {\n> +\t\t__u64 offset;\n> +\t\tstruct vfio_region_dma_tph tph;\n> +\t};\n> +\tunion {\n> +\t\t__u64 length;\n> +\t\t__u64 reserved;\n> +\t};\n> };\n> \n> struct vfio_device_feature_dma_buf {\n> @@ -1507,7 +1527,7 @@ struct vfio_device_feature_dma_buf {\n> \t__u32\topen_flags;\n> \t__u32 flags;\n> \t__u32 nr_ranges;\n> -\tstruct vfio_region_dma_range dma_ranges[] __counted_by(nr_ranges);\n> +\tstruct vfio_region_dma_range entries[];\n> };\n> \n> /* -------- API for Type1 VFIO IOMMU -------- */","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linux-pci@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=shazbot.org header.i=@shazbot.org header.a=rsa-sha256\n header.s=fm1 header.b=rUc2xiht;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=messagingengine.com header.i=@messagingengine.com\n header.a=rsa-sha256 header.s=fm2 header.b=QKfQ4zVP;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=172.234.253.10; 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x86_64-pc-linux-gnu)","Precedence":"bulk","X-Mailing-List":"linux-pci@vger.kernel.org","List-Id":"","List-Subscribe":"","List-Unsubscribe":"","MIME-Version":"1.0","Content-Type":"text/plain; charset=US-ASCII","Content-Transfer-Encoding":"7bit"}},{"id":3680692,"web_url":"http://patchwork.ozlabs.org/comment/3680692/","msgid":"<20260422162928.GL3611611@ziepe.ca>","list_archive_url":null,"date":"2026-04-22T16:29:28","subject":"Re: [PATCH v1 1/2] vfio: add callback to get tph info for dma-buf","submitter":{"id":73018,"url":"http://patchwork.ozlabs.org/api/people/73018/","name":"Jason Gunthorpe","email":"jgg@ziepe.ca"},"content":"On Wed, Apr 22, 2026 at 09:23:27AM -0600, Alex Williamson wrote:\n> In general though, I'm really hoping that someone interested in\n> enabling TPH as an interface through vfio actually decides to take\n> resource targeting and revocation seriously. There's no validation of\n> the steering tag here relative to what the user has access to and no\n> mechanism to revoke those tags if access changes. In fact, there's not\n> even a proposed mechanism allowing the user to derive valid steering\n> tags. Does the user implicitly know the value and the kernel just\n> allows it because... yolo? \n\nThis is the steering tag that remote devices will send *INTO* the VFIO\ndevice.\n\nIMHO it is entirely appropriate that the driver controlling the device\ndecide what tags are sent into it and when, so that's the VFIO\nuserspace.\n\nThere is no concept of access here since the entire device is captured\nby VFIO.\n\nIf the VFIO device catastrophically malfunctions when receiving\ncertain steering tags then it is incompatible with VFIO and we should\nat least block this new API..\n\nThe only requirement is that the device limit the TPH to only the\nfunction that is perceiving them. If a device is really broken and\ndoesn't meet that then it should be blocked off and it is probably not\nsafe to be used with VMs at all.\n\nJason","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linux-pci@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n secure) header.d=ziepe.ca header.i=@ziepe.ca header.a=rsa-sha256\n header.s=google header.b=pJvx2Y7f;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=172.232.135.74; helo=sto.lore.kernel.org;\n envelope-from=linux-pci+bounces-53011-incoming=patchwork.ozlabs.org@vger.kernel.org;\n receiver=patchwork.ozlabs.org)","smtp.subspace.kernel.org;\n\tdkim=pass (2048-bit key) header.d=ziepe.ca header.i=@ziepe.ca\n header.b=\"pJvx2Y7f\"","smtp.subspace.kernel.org;\n arc=none smtp.client-ip=209.85.222.172","smtp.subspace.kernel.org;\n dmarc=none (p=none dis=none) header.from=ziepe.ca","smtp.subspace.kernel.org;\n spf=pass smtp.mailfrom=ziepe.ca"],"Received":["from sto.lore.kernel.org (sto.lore.kernel.org [172.232.135.74])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4g14SJ2DHHz1yD5\n\tfor ; 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charset=us-ascii","Content-Disposition":"inline","In-Reply-To":"<20260422092327.3f629ad6@shazbot.org>"}},{"id":3680911,"web_url":"http://patchwork.ozlabs.org/comment/3680911/","msgid":"<20260422132740.5f809bf7@shazbot.org>","list_archive_url":null,"date":"2026-04-22T19:27:40","subject":"Re: [PATCH v1 1/2] vfio: add callback to get tph info for dma-buf","submitter":{"id":91887,"url":"http://patchwork.ozlabs.org/api/people/91887/","name":"Alex Williamson","email":"alex@shazbot.org"},"content":"On Wed, 22 Apr 2026 13:29:28 -0300\nJason Gunthorpe wrote:\n\n> On Wed, Apr 22, 2026 at 09:23:27AM -0600, Alex Williamson wrote:\n> > In general though, I'm really hoping that someone interested in\n> > enabling TPH as an interface through vfio actually decides to take\n> > resource targeting and revocation seriously. There's no validation of\n> > the steering tag here relative to what the user has access to and no\n> > mechanism to revoke those tags if access changes. In fact, there's not\n> > even a proposed mechanism allowing the user to derive valid steering\n> > tags. Does the user implicitly know the value and the kernel just\n> > allows it because... yolo? \n> \n> This is the steering tag that remote devices will send *INTO* the VFIO\n> device.\n> \n> IMHO it is entirely appropriate that the driver controlling the device\n> decide what tags are sent into it and when, so that's the VFIO\n> userspace.\n> \n> There is no concept of access here since the entire device is captured\n> by VFIO.\n> \n> If the VFIO device catastrophically malfunctions when receiving\n> certain steering tags then it is incompatible with VFIO and we should\n> at least block this new API..\n> \n> The only requirement is that the device limit the TPH to only the\n> function that is perceiving them. If a device is really broken and\n> doesn't meet that then it should be blocked off and it is probably not\n> safe to be used with VMs at all.\n\nOk, if the vfio user is only suggesting steering tags for another\ndriver to use when accessing their own device through the dma-buf, and\nthe lifecycle is bound to that dma-buf, maybe I'm overreacting on the\nsecurity aspect.\n\nI don't know how to qualify the statement in the last paragraph about\n\"[t]he only requirement is that the device limit the TPH to only the\nfunction that is perceiving them\", though. Is that implicit in being\nassociated to the dma-buf for the user owned device, or is it a\nproperty of the suggested steering tags, that we're not validating?\n\nSteering tags can induce caching abuse, as interpreted in the\ninterconnect fabric, but maybe we've already conceded that as\nfundamental aspect of TPH in general.\n\nSo why does vfio need to be involved in any of the sequence proposed\nhere? It seems like it would be a much cleaner design, avoiding\noverloading the existing vfio feature and questionable array semantics,\nif there were a set-tph ioctl on the resulting dma-buf instead of\nmaking some vfio specific interface bundling creation with tph hints.\nThanks,\n\nAlex","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linux-pci@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=shazbot.org header.i=@shazbot.org header.a=rsa-sha256\n header.s=fm1 header.b=H2zqWgVu;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=messagingengine.com header.i=@messagingengine.com\n header.a=rsa-sha256 header.s=fm2 header.b=JCcSbt3U;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=2600:3c04:e001:36c::12fc:5321; helo=tor.lore.kernel.org;\n envelope-from=linux-pci+bounces-53021-incoming=patchwork.ozlabs.org@vger.kernel.org;\n receiver=patchwork.ozlabs.org)","smtp.subspace.kernel.org;\n\tdkim=pass (2048-bit key) header.d=shazbot.org header.i=@shazbot.org\n header.b=\"H2zqWgVu\";\n\tdkim=pass (2048-bit key) header.d=messagingengine.com\n header.i=@messagingengine.com header.b=\"JCcSbt3U\"","smtp.subspace.kernel.org;\n arc=none smtp.client-ip=202.12.124.146","smtp.subspace.kernel.org;\n dmarc=pass (p=none dis=none) header.from=shazbot.org","smtp.subspace.kernel.org;\n spf=pass smtp.mailfrom=shazbot.org"],"Received":["from tor.lore.kernel.org (tor.lore.kernel.org\n [IPv6:2600:3c04:e001:36c::12fc:5321])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4g18Rv22d3z1yGs\n\tfor ; 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x86_64-pc-linux-gnu)","Precedence":"bulk","X-Mailing-List":"linux-pci@vger.kernel.org","List-Id":"","List-Subscribe":"","List-Unsubscribe":"","MIME-Version":"1.0","Content-Type":"text/plain; charset=US-ASCII","Content-Transfer-Encoding":"7bit"}},{"id":3681500,"web_url":"http://patchwork.ozlabs.org/comment/3681500/","msgid":"<20260423142828.GQ3611611@ziepe.ca>","list_archive_url":null,"date":"2026-04-23T14:28:28","subject":"Re: [PATCH v1 1/2] vfio: add callback to get tph info for dma-buf","submitter":{"id":73018,"url":"http://patchwork.ozlabs.org/api/people/73018/","name":"Jason Gunthorpe","email":"jgg@ziepe.ca"},"content":"On Wed, Apr 22, 2026 at 01:27:40PM -0600, Alex Williamson wrote:\n> I don't know how to qualify the statement in the last paragraph about\n> \"[t]he only requirement is that the device limit the TPH to only the\n> function that is perceiving them\", though. Is that implicit in being\n> associated to the dma-buf for the user owned device, or is it a\n> property of the suggested steering tags, that we're not validating?\n\nIt is a property of VFs and VFIO.\n\nFor instance if an insane device allows a steering tag to reach\noutside the VF's memory space then it can't really be used with VFIO.\n\n> Steering tags can induce caching abuse, as interpreted in the\n> interconnect fabric, but maybe we've already conceded that as\n> fundamental aspect of TPH in general.\n\nsteering tags are opaque, we don't know what a device will do when it\nreceives them.\n\nThe common CPU issue is indeed cache abuse, but who knows what a\ndevice will do with them.\n\n> So why does vfio need to be involved in any of the sequence proposed\n> here? It seems like it would be a much cleaner design, avoiding\n> overloading the existing vfio feature and questionable array semantics,\n> if there were a set-tph ioctl on the resulting dma-buf instead of\n> making some vfio specific interface bundling creation with tph\n> hints.\n\nRealistically only VFIO dmabufs will have this property that user\nspace can set any TPH.\n\nOther in-kernel drivers should accept some kind of hint from userspace\nwhen creating their dmabuf that makes sense for their device, not a\nraw TPH value. Like a GPU might accept a hint that specifies which\ndielet or something like that.\n\nSo I don't see a generality here from that perspective. The generality\nis that exporting drivers that can use TPH now have the option to tell\nthe importing driver to send them.\n\nJason","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linux-pci@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n secure) header.d=ziepe.ca header.i=@ziepe.ca header.a=rsa-sha256\n header.s=google header.b=IgRZtKjP;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=2600:3c0a:e001:db::12fc:5321; helo=sea.lore.kernel.org;\n envelope-from=linux-pci+bounces-53046-incoming=patchwork.ozlabs.org@vger.kernel.org;\n receiver=patchwork.ozlabs.org)","smtp.subspace.kernel.org;\n\tdkim=pass (2048-bit key) header.d=ziepe.ca header.i=@ziepe.ca\n header.b=\"IgRZtKjP\"","smtp.subspace.kernel.org;\n arc=none smtp.client-ip=209.85.222.172","smtp.subspace.kernel.org;\n dmarc=none (p=none dis=none) header.from=ziepe.ca","smtp.subspace.kernel.org;\n spf=pass smtp.mailfrom=ziepe.ca"],"Received":["from sea.lore.kernel.org (sea.lore.kernel.org\n [IPv6:2600:3c0a:e001:db::12fc:5321])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4g1dt765d7z1y2d\n\tfor ; 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charset=us-ascii","Content-Disposition":"inline","In-Reply-To":"<20260422132740.5f809bf7@shazbot.org>"}},{"id":3681649,"web_url":"http://patchwork.ozlabs.org/comment/3681649/","msgid":"<20260423132016.4a25e074@shazbot.org>","list_archive_url":null,"date":"2026-04-23T19:20:16","subject":"Re: [PATCH v1 1/2] vfio: add callback to get tph info for dma-buf","submitter":{"id":91887,"url":"http://patchwork.ozlabs.org/api/people/91887/","name":"Alex Williamson","email":"alex@shazbot.org"},"content":"On Thu, 23 Apr 2026 11:28:28 -0300\nJason Gunthorpe wrote:\n> On Wed, Apr 22, 2026 at 01:27:40PM -0600, Alex Williamson wrote:\n> \n> > So why does vfio need to be involved in any of the sequence proposed\n> > here? It seems like it would be a much cleaner design, avoiding\n> > overloading the existing vfio feature and questionable array semantics,\n> > if there were a set-tph ioctl on the resulting dma-buf instead of\n> > making some vfio specific interface bundling creation with tph\n> > hints. \n> \n> Realistically only VFIO dmabufs will have this property that user\n> space can set any TPH.\n> \n> Other in-kernel drivers should accept some kind of hint from userspace\n> when creating their dmabuf that makes sense for their device, not a\n> raw TPH value. Like a GPU might accept a hint that specifies which\n> dielet or something like that.\n> \n> So I don't see a generality here from that perspective. The generality\n> is that exporting drivers that can use TPH now have the option to tell\n> the importing driver to send them.\n\nOk, if dma_buf_ops.get_tph serves the common case of the driver\npresenting TPH values and vfio's case of the driver being only a\nconduit of user specified TPH values is unique, let's work on the vfio\nuAPI.\n\nWhy do we need to bundle dma-buf creation and TPH setting into a single\nioctl? That's what the proposal here does and it results in a really\nugly extension with an off-by-one baked into it.\n\nMy suggestion would be that we leave VFIO_DEVICE_FEATURE_DMA_BUF\nunchanged and add a VFIO_DEVICE_FEATURE_DMA_BUF_TPH ioctl which takes\nthe fd from VFIO_DEVICE_FEATURE_DMA_BUF, along with a steering tag and\nprocessing hint. It would fdget() the dmabuf fd, validate it's a\ndmabuf via f_ops, validate it's a vfio exported dmabuf via dmabuf->ops,\nfind the matching vfio_pci_dma_buf via priv under memory_lock, and\nstuff the provided TPH values into the object. It would be left to the\nuser to sequence setting the TPH values on the dmabuf before the dmabuf\nis consumed by the importer.\n\nIs that a more reasonable uAPI? Thanks,\n\nAlex","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linux-pci@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=shazbot.org header.i=@shazbot.org header.a=rsa-sha256\n header.s=fm1 header.b=gdx1ISCQ;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=messagingengine.com header.i=@messagingengine.com\n header.a=rsa-sha256 header.s=fm2 header.b=ZUvdr8y6;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=2600:3c04:e001:36c::12fc:5321; helo=tor.lore.kernel.org;\n envelope-from=linux-pci+bounces-53064-incoming=patchwork.ozlabs.org@vger.kernel.org;\n receiver=patchwork.ozlabs.org)","smtp.subspace.kernel.org;\n\tdkim=pass (2048-bit key) header.d=shazbot.org header.i=@shazbot.org\n header.b=\"gdx1ISCQ\";\n\tdkim=pass (2048-bit key) header.d=messagingengine.com\n header.i=@messagingengine.com header.b=\"ZUvdr8y6\"","smtp.subspace.kernel.org;\n arc=none smtp.client-ip=103.168.172.153","smtp.subspace.kernel.org;\n dmarc=pass (p=none dis=none) header.from=shazbot.org","smtp.subspace.kernel.org;\n spf=pass smtp.mailfrom=shazbot.org"],"Received":["from tor.lore.kernel.org (tor.lore.kernel.org\n [IPv6:2600:3c04:e001:36c::12fc:5321])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4g1mHr2qxQz1yCv\n\tfor ; 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x86_64-pc-linux-gnu)","Precedence":"bulk","X-Mailing-List":"linux-pci@vger.kernel.org","List-Id":"","List-Subscribe":"","List-Unsubscribe":"","MIME-Version":"1.0","Content-Type":"text/plain; charset=US-ASCII","Content-Transfer-Encoding":"7bit"}},{"id":3681706,"web_url":"http://patchwork.ozlabs.org/comment/3681706/","msgid":"<20260423224626.GV3611611@ziepe.ca>","list_archive_url":null,"date":"2026-04-23T22:46:26","subject":"Re: [PATCH v1 1/2] vfio: add callback to get tph info for dma-buf","submitter":{"id":73018,"url":"http://patchwork.ozlabs.org/api/people/73018/","name":"Jason Gunthorpe","email":"jgg@ziepe.ca"},"content":"On Thu, Apr 23, 2026 at 01:20:16PM -0600, Alex Williamson wrote:\n\n> My suggestion would be that we leave VFIO_DEVICE_FEATURE_DMA_BUF\n> unchanged and add a VFIO_DEVICE_FEATURE_DMA_BUF_TPH ioctl which takes\n> the fd from VFIO_DEVICE_FEATURE_DMA_BUF, along with a steering tag and\n> processing hint. It would fdget() the dmabuf fd, validate it's a\n> dmabuf via f_ops, validate it's a vfio exported dmabuf via dmabuf->ops,\n> find the matching vfio_pci_dma_buf via priv under memory_lock, and\n> stuff the provided TPH values into the object. It would be left to the\n> user to sequence setting the TPH values on the dmabuf before the dmabuf\n> is consumed by the importer.\n> \n> Is that a more reasonable uAPI? Thanks,\n\nOff hand I think it can work, with the proviso that if userspace uses\nthe dmabuf before setting the tph the importer may ignore it. I don't\nthink that is a problem in practice.\n\nJason","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linux-pci@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n secure) header.d=ziepe.ca header.i=@ziepe.ca header.a=rsa-sha256\n header.s=google header.b=gHsocES4;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=2600:3c0a:e001:db::12fc:5321; helo=sea.lore.kernel.org;\n envelope-from=linux-pci+bounces-53082-incoming=patchwork.ozlabs.org@vger.kernel.org;\n receiver=patchwork.ozlabs.org)","smtp.subspace.kernel.org;\n\tdkim=pass (2048-bit key) header.d=ziepe.ca header.i=@ziepe.ca\n header.b=\"gHsocES4\"","smtp.subspace.kernel.org;\n arc=none smtp.client-ip=209.85.222.171","smtp.subspace.kernel.org;\n dmarc=none (p=none dis=none) header.from=ziepe.ca","smtp.subspace.kernel.org;\n spf=pass smtp.mailfrom=ziepe.ca"],"Received":["from sea.lore.kernel.org (sea.lore.kernel.org\n [IPv6:2600:3c0a:e001:db::12fc:5321])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519 server-signature ECDSA (secp384r1) server-digest SHA384)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4g1rmn0MTPz1yDD\n\tfor ; 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charset=us-ascii","Content-Disposition":"inline","In-Reply-To":"<20260423132016.4a25e074@shazbot.org>"}},{"id":3681774,"web_url":"http://patchwork.ozlabs.org/comment/3681774/","msgid":"","list_archive_url":null,"date":"2026-04-24T05:41:28","subject":"Re: [PATCH v1 1/2] vfio: add callback to get tph info for dma-buf","submitter":{"id":92088,"url":"http://patchwork.ozlabs.org/api/people/92088/","name":"Zhiping Zhang","email":"zhipingz@meta.com"},"content":"On Thu, Apr 23, 2026 at 3:46 PM Jason Gunthorpe wrote:\n>\n> >\n> On Thu, Apr 23, 2026 at 01:20:16PM -0600, Alex Williamson wrote:\n>\n> > My suggestion would be that we leave VFIO_DEVICE_FEATURE_DMA_BUF\n> > unchanged and add a VFIO_DEVICE_FEATURE_DMA_BUF_TPH ioctl which takes\n> > the fd from VFIO_DEVICE_FEATURE_DMA_BUF, along with a steering tag and\n> > processing hint. It would fdget() the dmabuf fd, validate it's a\n> > dmabuf via f_ops, validate it's a vfio exported dmabuf via dmabuf->ops,\n> > find the matching vfio_pci_dma_buf via priv under memory_lock, and\n> > stuff the provided TPH values into the object. It would be left to the\n> > user to sequence setting the TPH values on the dmabuf before the dmabuf\n> > is consumed by the importer.\n> >\n> > Is that a more reasonable uAPI? Thanks,\n>\n> Off hand I think it can work, with the proviso that if userspace uses\n> the dmabuf before setting the tph the importer may ignore it. I don't\n> think that is a problem in practice.\n>\n> Jason\n\nThanks Alex and Jason!\n\nI agree that the separate feature sounds like a cleaner design and could avoid\nthe uAPI complications. For v2 I'll:\n - Leave VFIO_DEVICE_FEATURE_DMA_BUF unchanged,\n - Add VFIO_DEVICE_FEATURE_DMA_BUF_TPH as a SET-only feature that\n takes the dmabuf fd, steering tag, and processing hint,\n - Keep the dma_buf_ops.get_tph callback as-is.\nand then the userspace sequence would become:\n1. fd = VFIO_DEVICE_FEATURE_DMA_BUF (create dmabuf, unchanged)\n2. VFIO_DEVICE_FEATURE_DMA_BUF_TPH (set TPH on fd)\n3. pass fd to importer\n\nZhiping","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linux-pci@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=meta.com header.i=@meta.com header.a=rsa-sha256\n header.s=s2048-2025-q2 header.b=Lm/Az7wf;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=2600:3c0a:e001:db::12fc:5321; helo=sea.lore.kernel.org;\n envelope-from=linux-pci+bounces-53102-incoming=patchwork.ozlabs.org@vger.kernel.org;\n receiver=patchwork.ozlabs.org)","smtp.subspace.kernel.org;\n\tdkim=pass (2048-bit key) header.d=meta.com header.i=@meta.com\n header.b=\"Lm/Az7wf\"","smtp.subspace.kernel.org;\n arc=fail smtp.client-ip=67.231.153.30","smtp.subspace.kernel.org;\n dmarc=pass (p=reject dis=none) header.from=meta.com","smtp.subspace.kernel.org;\n spf=pass smtp.mailfrom=meta.com"],"Received":["from sea.lore.kernel.org (sea.lore.kernel.org\n [IPv6:2600:3c0a:e001:db::12fc:5321])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519 server-signature ECDSA (secp384r1) server-digest SHA384)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4g22125kR9z1xvV\n\tfor ; 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