[{"id":3675291,"web_url":"http://patchwork.ozlabs.org/comment/3675291/","msgid":"","list_archive_url":null,"date":"2026-04-09T12:33:44","subject":"Re: [PATCH] vfio: Add VFIO I/O backend capability flags for feature\n support","submitter":{"id":80631,"url":"http://patchwork.ozlabs.org/api/people/80631/","name":"John Levon","email":"john.levon@nutanix.com"},"content":"On Thu, Apr 09, 2026 at 01:43:12PM +0200, Cédric Le Goater wrote:\n\n> !-------------------------------------------------------------------|\n> CAUTION: External Email\n> \n> |-------------------------------------------------------------------!\n> \n> Different VFIO I/O backends support different features. For example,\n> the kernel VFIO backend supports DMA-BUF creation, while vfio-user\n> does not. Currently, this is handled by attempting the operation and\n> checking for -ENOTTY, which can lead to misleading warnings when a\n> feature is simply not supported by a particular backend.\n> \n> Introduce a capability flags mechanism in VFIODeviceIOOps that allows\n> backends to explicitly advertise which features they support. Callers\n> can check these capabilities before attempting operations, avoiding\n> spurious errors and warnings.\n> \n> Cc: John Levon \n> Signed-off-by: Cédric Le Goater \n\nThanks!\n\nReviewed-by: John Levon \n\nregards\njohn","headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=nutanix.com header.i=@nutanix.com header.a=rsa-sha256\n header.s=proofpoint20171006 header.b=bSiZd2N3;\n\tdkim=fail reason=\"signature verification failed\" (2048-bit key;\n unprotected) header.d=nutanix.com header.i=@nutanix.com header.a=rsa-sha256\n header.s=selector1 header.b=gYSeFed2;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; helo=lists.gnu.org;\n envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org;\n 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SPF_PASS=-0.001 autolearn=ham autolearn_force=no","X-Spam_action":"no action","X-BeenThere":"qemu-devel@nongnu.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"qemu development ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org","Sender":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org"}},{"id":3675683,"web_url":"http://patchwork.ozlabs.org/comment/3675683/","msgid":"","list_archive_url":null,"date":"2026-04-10T07:00:06","subject":"Re: [PATCH] vfio: Add VFIO I/O backend capability flags for feature\n support","submitter":{"id":85406,"url":"http://patchwork.ozlabs.org/api/people/85406/","name":"Cédric Le Goater","email":"clg@redhat.com"},"content":"On 4/9/26 13:43, Cédric Le Goater wrote:\n> Different VFIO I/O backends support different features. For example,\n> the kernel VFIO backend supports DMA-BUF creation, while vfio-user\n> does not. Currently, this is handled by attempting the operation and\n> checking for -ENOTTY, which can lead to misleading warnings when a\n> feature is simply not supported by a particular backend.\n> \n> Introduce a capability flags mechanism in VFIODeviceIOOps that allows\n> backends to explicitly advertise which features they support. Callers\n> can check these capabilities before attempting operations, avoiding\n> spurious errors and warnings.\n> \n> Cc: John Levon \n> Signed-off-by: Cédric Le Goater \n> ---\n> include/hw/vfio/vfio-device.h | 13 +++++++++++++\n> hw/vfio/device.c | 2 ++\n> hw/vfio/region.c | 5 +++++\n> 3 files changed, 20 insertions(+)\n> \n> diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h\n> index a95c5bf50302a7394582dcc493d96aef00b47dc8..380a55d6e5ea280c8f44016c7a8a1dcb8f4c00fe 100644\n> --- a/include/hw/vfio/vfio-device.h\n> +++ b/include/hw/vfio/vfio-device.h\n> @@ -172,12 +172,25 @@ typedef QLIST_HEAD(VFIODeviceList, VFIODevice) VFIODeviceList;\n> extern VFIODeviceList vfio_device_list;\n> \n> #ifdef CONFIG_LINUX\n> +/*\n> + * VFIO backend I/O operation capabilities\n> + */\n> +#define VFIO_IO_CAP_DMA_BUF (1ULL << 0)\n> +\n> /*\n> * How devices communicate with the server. The default option is through\n> * ioctl() to the kernel VFIO driver, but vfio-user can use a socket to a remote\n> * process.\n> */\n> struct VFIODeviceIOOps {\n> + /**\n> + * @capabilities\n> + *\n> + * Bitmask of VFIO_IO_CAP_* flags indicating which features this\n> + * backend supports.\n> + */\n> + uint64_t capabilities;\n> +\n> /**\n> * @device_feature\n> *\n> diff --git a/hw/vfio/device.c b/hw/vfio/device.c\n> index 8f7ae919a55dd9e08bdd072eee33f5b46a91aa2a..3ffd69a579b8ac135ef729d3d836c8545e36786b 100644\n> --- a/hw/vfio/device.c\n> +++ b/hw/vfio/device.c\n> @@ -648,6 +648,8 @@ static int vfio_device_io_region_write(VFIODevice *vbasedev, uint8_t index,\n> }\n> \n> static VFIODeviceIOOps vfio_device_io_ops_ioctl = {\n> + .capabilities = VFIO_IO_CAP_DMA_BUF,\n> +\n> .device_feature = vfio_device_io_device_feature,\n> .get_region_info = vfio_device_io_get_region_info,\n> .get_irq_info = vfio_device_io_get_irq_info,\n> diff --git a/hw/vfio/region.c b/hw/vfio/region.c\n> index 47fdc2df349b65c6be6c9605b7a38a4e367f0475..0342ca712acd0ad846685115c29432c15aa966a0 100644\n> --- a/hw/vfio/region.c\n> +++ b/hw/vfio/region.c\n> @@ -293,6 +293,11 @@ static bool vfio_region_create_dma_buf(VFIORegion *region, Error **errp)\n> size_t total_size;\n> int i, ret;\n> \n> + /* Check if backend supports DMA-BUF creation */\n> + if (!(vbasedev->io_ops->capabilities & VFIO_IO_CAP_DMA_BUF)) {\n> + return true;\n> + }\n> +\n> total_size = sizeof(*feature) + sizeof(*dma_buf) +\n> sizeof(struct vfio_region_dma_range) * region->nr_mmaps;\n> feature = g_malloc0(total_size);\n\nApplied to vfio-next.\n\nThanks,\n\nC.","headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=fuHO91l2;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=google header.b=YsQGNcr2;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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