[{"id":3683942,"web_url":"http://patchwork.ozlabs.org/comment/3683942/","msgid":"","list_archive_url":null,"date":"2026-04-29T09:18:41","subject":"Re: [Intel-wired-lan] [PATCH iwl-next v3] e1000e: Avoid DMA\n re-mapping on RX copybreak","submitter":{"id":75597,"url":"http://patchwork.ozlabs.org/api/people/75597/","name":"Aleksandr Loktionov","email":"aleksandr.loktionov@intel.com"},"content":"> -----Original Message-----\n> From: Matt Vollrath \n> Sent: Wednesday, April 29, 2026 3:43 AM\n> To: intel-wired-lan@osuosl.org\n> Cc: Matt Vollrath ; Loktionov, Aleksandr\n> ; Paul Menzel \n> Subject: [PATCH iwl-next v3] e1000e: Avoid DMA re-mapping on RX\n> copybreak\n> \n> This patch factors out DMA re-mapping for skbs which were recycled in\n> the RX path due to copybreak or errors. There is only one path out of\n> the e1000_clean_rx_irq() loop where the skb is consumed and DMA needs\n> to be re-mapped, so don't unmap it before checking the conditions.\n> \n> The buffer allocation loop is adjusted to not assume that DMA is\n> unmapped, handling mapping errors gracefully.\n> \n> On systems with IOMMU enabled, the cost of re-mapping DMA is greater\n> than the cost of copying data out of the ring buffer. When I use this\n> patch and configure e1000e with copybreak=2048, my system with IOMMU\n> completes RX roughly twice as fast under load.\n> \n> Informal performance comparisons were based on Asus Gryphon Z97 which\n> includes an I218-V and with a Xeon E3-1240 v3 in the socket.\n> ktime_get() measurement was injected into e1000e_poll() wrapping the\n> adapter->clean_rx() call. The total time spent in clean_rx() was\n> divided\n> by work_done to print the average time spent per buffer. iperf3 -R was\n> used to saturate the RX path and awk was used for statistics. Control\n> revision was set to 7.1-rc1 because iwl-next hadn't been updated yet.\n> \n> rev | iommu | copybreak | samples | mean (ns) | stdev\n> 7.1-rc1 | off | 0 | 4748 | 453.72 | 155.82\n> 7.1-rc1 | off | 2048 | 4743 | 554.83 | 103.67\n> 7.1-rc1 | on | 0 | 4751 | 1139.22 | 150.56\n> * 7.1-rc1 | on | 2048 | 4737 | 1267.02 | 184.62\n> +patch | off | 0 | 4739 | 456.30 | 146.33\n> +patch | off | 2048 | 4739 | 538.56 | 132.97\n> +patch | on | 0 | 4769 | 1165.97 | 140.19\n> * +patch | on | 2048 | 4745 | 562.25 | 171.80\n> \n> No surprises here, IOMMU DMA ops are known to be expensive. For most\n> users the kernel default is iommu=on and driver default is\n> copybreak=256, so unless the workload is small packets, some tuning of\n> either knob would be needed to see the full benefit of this change.\n> \n> The kludge of unconditional unmapping has existed since this driver\n> was introduced in 2007[1], inherited from the e1000 driver which has\n> since factored it out[2]. IOMMU tech was new at the time.\n> \n> [1] Commit bc7f75fa9788 (\"[E1000E]: New pci-express e1000 driver\n> (currently for ICH9 devices only)\") [2] Commit 2b294b18689c (\"e1000:\n> perform copybreak ahead of DMA unmap\")\n> \n> Assisted-by: Claude:claude-4-7-opus\n> Signed-off-by: Matt Vollrath \n> ---\n> v3:\n> * refactor unmapping bypass, use goto instead of redundant branch\n> * remove Aleksandr's sign-off due to logic change\n> * benchmark details\n> * cite historic commits\n> v2:\n> * proofread description with Aleksandr\n> ---\n> drivers/net/ethernet/intel/e1000e/netdev.c | 32 ++++++++++++++-------\n> -\n> 1 file changed, 21 insertions(+), 11 deletions(-)\n> \n> diff --git a/drivers/net/ethernet/intel/e1000e/netdev.c\n> b/drivers/net/ethernet/intel/e1000e/netdev.c\n> index 7ce0cc8ab8f4..62bf85c768d6 100644\n> --- a/drivers/net/ethernet/intel/e1000e/netdev.c\n> +++ b/drivers/net/ethernet/intel/e1000e/netdev.c\n> @@ -663,6 +663,8 @@ static void e1000_alloc_rx_buffers(struct\n> e1000_ring *rx_ring,\n> \t\tskb = buffer_info->skb;\n> \t\tif (skb) {\n> \t\t\tskb_trim(skb, 0);\n> +\t\t\tif (likely(buffer_info->dma))\n> +\t\t\t\tgoto write_desc;\n> \t\t\tgoto map_skb;\n> \t\t}\n> \n> @@ -680,10 +682,12 @@ static void e1000_alloc_rx_buffers(struct\n> e1000_ring *rx_ring,\n> \t\t\t\t\t\t DMA_FROM_DEVICE);\n> \t\tif (dma_mapping_error(&pdev->dev, buffer_info->dma)) {\n> \t\t\tdev_err(&pdev->dev, \"Rx DMA map failed\\n\");\n> +\t\t\tbuffer_info->dma = 0;\n> \t\t\tadapter->rx_dma_failed++;\n> \t\t\tbreak;\n> \t\t}\n> \n> +write_desc:\n> \t\trx_desc = E1000_RX_DESC_EXT(*rx_ring, i);\n> \t\trx_desc->read.buffer_addr = cpu_to_le64(buffer_info-\n> >dma);\n> \n> @@ -941,7 +945,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring\n> *rx_ring, int *work_done,\n> \t\tdma_rmb();\t/* read descriptor and rx_buffer_info after\n> status DD */\n> \n> \t\tskb = buffer_info->skb;\n> -\t\tbuffer_info->skb = NULL;\n> \n> \t\tprefetch(skb->data - NET_IP_ALIGN);\n> \n> @@ -955,9 +958,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring\n> *rx_ring, int *work_done,\n> \n> \t\tcleaned = true;\n> \t\tcleaned_count++;\n> -\t\tdma_unmap_single(&pdev->dev, buffer_info->dma,\n> -\t\t\t\t adapter->rx_buffer_len, DMA_FROM_DEVICE);\n> -\t\tbuffer_info->dma = 0;\n> \n> \t\tlength = le16_to_cpu(rx_desc->wb.upper.length);\n> \n> @@ -973,8 +973,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring\n> *rx_ring, int *work_done,\n> \t\tif (adapter->flags2 & FLAG2_IS_DISCARDING) {\n> \t\t\t/* All receives must fit into a single buffer */\n> \t\t\te_dbg(\"Receive packet consumed multiple\n> buffers\\n\");\n> -\t\t\t/* recycle */\n> -\t\t\tbuffer_info->skb = skb;\n> \t\t\tif (staterr & E1000_RXD_STAT_EOP)\n> \t\t\t\tadapter->flags2 &= ~FLAG2_IS_DISCARDING;\n> \t\t\tgoto next_desc;\n> @@ -982,8 +980,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring\n> *rx_ring, int *work_done,\n> \n> \t\tif (unlikely((staterr & E1000_RXDEXT_ERR_FRAME_ERR_MASK)\n> &&\n> \t\t\t !(netdev->features & NETIF_F_RXALL))) {\n> -\t\t\t/* recycle */\n> -\t\t\tbuffer_info->skb = skb;\n> \t\t\tgoto next_desc;\n> \t\t}\n> \n> @@ -1010,19 +1006,33 @@ static bool e1000_clean_rx_irq(struct\n> e1000_ring *rx_ring, int *work_done,\n> \t\t\tstruct sk_buff *new_skb =\n> \t\t\t\tnapi_alloc_skb(&adapter->napi, length);\n> \t\t\tif (new_skb) {\n> +\t\t\t\tdma_sync_single_for_cpu(&pdev->dev,\n> +\t\t\t\t\t\t\tbuffer_info->dma,\n> +\t\t\t\t\t\t\tadapter-\n> >rx_buffer_len,\n> +\t\t\t\t\t\t\tDMA_FROM_DEVICE);\n> \t\t\t\tskb_copy_to_linear_data_offset(new_skb,\n> \t\t\t\t\t\t\t -NET_IP_ALIGN,\n> \t\t\t\t\t\t\t (skb->data -\n> \t\t\t\t\t\t\t\tNET_IP_ALIGN),\n> \t\t\t\t\t\t\t (length +\n> \t\t\t\t\t\t\t\tNET_IP_ALIGN));\n> -\t\t\t\t/* save the skb in buffer_info as good */\n> -\t\t\t\tbuffer_info->skb = skb;\n> +\t\t\t\tdma_sync_single_for_device(&pdev->dev,\n> +\t\t\t\t\t\t\t buffer_info->dma,\n> +\t\t\t\t\t\t\t adapter-\n> >rx_buffer_len,\n> +\t\t\t\t\t\t\t DMA_FROM_DEVICE);\n> \t\t\t\tskb = new_skb;\n> +\t\t\t\tgoto copybreak_done;\n> \t\t\t}\n> \t\t\t/* else just continue with the old one */\n> \t\t}\n> -\t\t/* end copybreak code */\n> +\n> +\t\tbuffer_info->skb = NULL;\n> +\t\tdma_unmap_single(&pdev->dev, buffer_info->dma,\n> +\t\t\t\t adapter->rx_buffer_len,\n> +\t\t\t\t DMA_FROM_DEVICE);\n> +\t\tbuffer_info->dma = 0;\n> +\n> +copybreak_done:\n> \t\tskb_put(skb, length);\n> \n> \t\t/* Receive Checksum Offload */\n> --\n> 2.43.0\n\nReviewed-by: Aleksandr Loktionov ","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","intel-wired-lan@lists.osuosl.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","intel-wired-lan@lists.osuosl.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=osuosl.org header.i=@osuosl.org header.a=rsa-sha256\n header.s=default header.b=Tp/1asCV;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=osuosl.org\n (client-ip=2605:bc80:3010::136; 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a=rsa-sha256; c=relaxed/simple;\n d=intel.com; i=@intel.com; q=dns/txt; s=Intel;\n t=1777454329; x=1808990329;\n h=from:to:cc:subject:date:message-id:references:\n in-reply-to:content-transfer-encoding:mime-version;\n bh=sICU8YS8e/ZECNTBdRtm9vErF5kE2sbGi1mdEyJTU8w=;\n b=Lmh2vPAmYjXDvqGrUG23gOfnZ67wEcWjzCdD4W1UpFABVc62skLIiYnL\n 3PpxjvwplJ7PqBf/5+UEIj5rh0Grbd+rKQCuWwOMWCJogJ5rBiruY2IV6\n yZNo+/CjYosQzubwhX3gcGN5w8CNLqWv/kxLNz+G+MDpAD+C5Fd05XNem\n fkRqDBtdnLYhDaDghq9UGn5fh9wZ/u5yoAvyWUkluBbzwMMml2xBlWWZt\n G3VFp1m1ZD9ODlXNM2ckc+anOyH+JFgxPSauNA7ju7fxJbQ35nNfV3gLw\n U2xCzYHgd3s1K0ZMLA9xvE9yZtiPGT5OiBywYdHVfBXfEntSoZHC+PJZP\n g==;","X-Mailman-Original-Authentication-Results":["smtp1.osuosl.org;\n dmarc=pass (p=none dis=none)\n header.from=intel.com","smtp1.osuosl.org;\n dkim=pass (2048-bit key,\n unprotected) header.d=intel.com header.i=@intel.com header.a=rsa-sha256\n header.s=Intel header.b=Lmh2vPAm","dkim=none (message not signed)\n header.d=none;dmarc=none action=none header.from=intel.com;"],"Subject":"Re: [Intel-wired-lan] [PATCH iwl-next v3] e1000e: Avoid DMA\n re-mapping on RX copybreak","X-BeenThere":"intel-wired-lan@osuosl.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Intel Wired Ethernet Linux Kernel Driver Development\n ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"intel-wired-lan-bounces@osuosl.org","Sender":"\"Intel-wired-lan\" "}},{"id":3683982,"web_url":"http://patchwork.ozlabs.org/comment/3683982/","msgid":"<89e43943-d114-4de5-8fec-efabc3cdc20e@molgen.mpg.de>","list_archive_url":null,"date":"2026-04-29T10:32:52","subject":"Re: [Intel-wired-lan] [PATCH iwl-next v3] e1000e: Avoid DMA\n re-mapping on RX copybreak","submitter":{"id":70275,"url":"http://patchwork.ozlabs.org/api/people/70275/","name":"Paul Menzel","email":"pmenzel@molgen.mpg.de"},"content":"Dear Matt,\n\n\nThank you for this iteration.\n\nAm 29.04.26 um 03:43 schrieb Matt Vollrath:\n> This patch factors out DMA re-mapping for skbs which were recycled in\n> the RX path due to copybreak or errors. There is only one path out of\n> the e1000_clean_rx_irq() loop where the skb is consumed and DMA needs\n> to be re-mapped, so don't unmap it before checking the conditions.\n> \n> The buffer allocation loop is adjusted to not assume that DMA is\n> unmapped, handling mapping errors gracefully.\n> \n> On systems with IOMMU enabled, the cost of re-mapping DMA is greater\n> than the cost of copying data out of the ring buffer. When I use this\n> patch and configure e1000e with copybreak=2048, my system with IOMMU\n> completes RX roughly twice as fast under load.\n> \n> Informal performance comparisons were based on Asus Gryphon Z97 which\n> includes an I218-V and with a Xeon E3-1240 v3 in the socket. ktime_get()\n> measurement was injected into e1000e_poll() wrapping the\n> adapter->clean_rx() call. The total time spent in clean_rx() was divided\n> by work_done to print the average time spent per buffer. iperf3 -R was\n> used to saturate the RX path and awk was used for statistics. Control\n> revision was set to 7.1-rc1 because iwl-next hadn't been updated yet.\n> \n> rev | iommu | copybreak | samples | mean (ns) | stdev\n> 7.1-rc1 | off | 0 | 4748 | 453.72 | 155.82\n> 7.1-rc1 | off | 2048 | 4743 | 554.83 | 103.67\n> 7.1-rc1 | on | 0 | 4751 | 1139.22 | 150.56\n> * 7.1-rc1 | on | 2048 | 4737 | 1267.02 | 184.62\n> +patch | off | 0 | 4739 | 456.30 | 146.33\n> +patch | off | 2048 | 4739 | 538.56 | 132.97\n> +patch | on | 0 | 4769 | 1165.97 | 140.19\n> * +patch | on | 2048 | 4745 | 562.25 | 171.80\n\nAwesome description and overview. Thank you! Values for the default case \nof `copybreak=256` would be nice to have. But no need to resend for my sake.\n\n> No surprises here, IOMMU DMA ops are known to be expensive. For most\n> users the kernel default is iommu=on and driver default is\n> copybreak=256, so unless the workload is small packets, some tuning of\n> either knob would be needed to see the full benefit of this change.\n> \n> The kludge of unconditional unmapping has existed since this driver was\n> introduced in 2007[1], inherited from the e1000 driver which has since\n> factored it out[2]. IOMMU tech was new at the time.\n> \n> [1] Commit bc7f75fa9788 (\"[E1000E]: New pci-express e1000 driver (currently for ICH9 devices only)\")\n> [2] Commit 2b294b18689c (\"e1000: perform copybreak ahead of DMA unmap\")\n> \n> Assisted-by: Claude:claude-4-7-opus\n> Signed-off-by: Matt Vollrath \n> ---\n> v3:\n> * refactor unmapping bypass, use goto instead of redundant branch\n> * remove Aleksandr's sign-off due to logic change\n> * benchmark details\n> * cite historic commits\n> v2:\n> * proofread description with Aleksandr\n> ---\n> drivers/net/ethernet/intel/e1000e/netdev.c | 32 ++++++++++++++--------\n> 1 file changed, 21 insertions(+), 11 deletions(-)\n> \n> diff --git a/drivers/net/ethernet/intel/e1000e/netdev.c b/drivers/net/ethernet/intel/e1000e/netdev.c\n> index 7ce0cc8ab8f4..62bf85c768d6 100644\n> --- a/drivers/net/ethernet/intel/e1000e/netdev.c\n> +++ b/drivers/net/ethernet/intel/e1000e/netdev.c\n> @@ -663,6 +663,8 @@ static void e1000_alloc_rx_buffers(struct e1000_ring *rx_ring,\n> \t\tskb = buffer_info->skb;\n> \t\tif (skb) {\n> \t\t\tskb_trim(skb, 0);\n> +\t\t\tif (likely(buffer_info->dma))\n> +\t\t\t\tgoto write_desc;\n> \t\t\tgoto map_skb;\n> \t\t}\n> \n> @@ -680,10 +682,12 @@ static void e1000_alloc_rx_buffers(struct e1000_ring *rx_ring,\n> \t\t\t\t\t\t DMA_FROM_DEVICE);\n> \t\tif (dma_mapping_error(&pdev->dev, buffer_info->dma)) {\n> \t\t\tdev_err(&pdev->dev, \"Rx DMA map failed\\n\");\n> +\t\t\tbuffer_info->dma = 0;\n> \t\t\tadapter->rx_dma_failed++;\n> \t\t\tbreak;\n> \t\t}\n> \n> +write_desc:\n> \t\trx_desc = E1000_RX_DESC_EXT(*rx_ring, i);\n> \t\trx_desc->read.buffer_addr = cpu_to_le64(buffer_info->dma);\n> \n> @@ -941,7 +945,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,\n> \t\tdma_rmb();\t/* read descriptor and rx_buffer_info after status DD */\n> \n> \t\tskb = buffer_info->skb;\n> -\t\tbuffer_info->skb = NULL;\n> \n> \t\tprefetch(skb->data - NET_IP_ALIGN);\n> \n> @@ -955,9 +958,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,\n> \n> \t\tcleaned = true;\n> \t\tcleaned_count++;\n> -\t\tdma_unmap_single(&pdev->dev, buffer_info->dma,\n> -\t\t\t\t adapter->rx_buffer_len, DMA_FROM_DEVICE);\n> -\t\tbuffer_info->dma = 0;\n> \n> \t\tlength = le16_to_cpu(rx_desc->wb.upper.length);\n> \n> @@ -973,8 +973,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,\n> \t\tif (adapter->flags2 & FLAG2_IS_DISCARDING) {\n> \t\t\t/* All receives must fit into a single buffer */\n> \t\t\te_dbg(\"Receive packet consumed multiple buffers\\n\");\n> -\t\t\t/* recycle */\n> -\t\t\tbuffer_info->skb = skb;\n> \t\t\tif (staterr & E1000_RXD_STAT_EOP)\n> \t\t\t\tadapter->flags2 &= ~FLAG2_IS_DISCARDING;\n> \t\t\tgoto next_desc;\n> @@ -982,8 +980,6 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,\n> \n> \t\tif (unlikely((staterr & E1000_RXDEXT_ERR_FRAME_ERR_MASK) &&\n> \t\t\t !(netdev->features & NETIF_F_RXALL))) {\n> -\t\t\t/* recycle */\n> -\t\t\tbuffer_info->skb = skb;\n> \t\t\tgoto next_desc;\n> \t\t}\n> \n> @@ -1010,19 +1006,33 @@ static bool e1000_clean_rx_irq(struct e1000_ring *rx_ring, int *work_done,\n> \t\t\tstruct sk_buff *new_skb =\n> \t\t\t\tnapi_alloc_skb(&adapter->napi, length);\n> \t\t\tif (new_skb) {\n> +\t\t\t\tdma_sync_single_for_cpu(&pdev->dev,\n> +\t\t\t\t\t\t\tbuffer_info->dma,\n> +\t\t\t\t\t\t\tadapter->rx_buffer_len,\n> +\t\t\t\t\t\t\tDMA_FROM_DEVICE);\n> \t\t\t\tskb_copy_to_linear_data_offset(new_skb,\n> \t\t\t\t\t\t\t -NET_IP_ALIGN,\n> \t\t\t\t\t\t\t (skb->data -\n> \t\t\t\t\t\t\t\tNET_IP_ALIGN),\n> \t\t\t\t\t\t\t (length +\n> \t\t\t\t\t\t\t\tNET_IP_ALIGN));\n> -\t\t\t\t/* save the skb in buffer_info as good */\n> -\t\t\t\tbuffer_info->skb = skb;\n> +\t\t\t\tdma_sync_single_for_device(&pdev->dev,\n> +\t\t\t\t\t\t\t buffer_info->dma,\n> +\t\t\t\t\t\t\t adapter->rx_buffer_len,\n> +\t\t\t\t\t\t\t DMA_FROM_DEVICE);\n> \t\t\t\tskb = new_skb;\n> +\t\t\t\tgoto copybreak_done;\n> \t\t\t}\n> \t\t\t/* else just continue with the old one */\n> \t\t}\n> -\t\t/* end copybreak code */\n> +\n> +\t\tbuffer_info->skb = NULL;\n> +\t\tdma_unmap_single(&pdev->dev, buffer_info->dma,\n> +\t\t\t\t adapter->rx_buffer_len,\n> +\t\t\t\t DMA_FROM_DEVICE);\n> +\t\tbuffer_info->dma = 0;\n> +\n> +copybreak_done:\n> \t\tskb_put(skb, length);\n> \n> \t\t/* Receive Checksum Offload */\n\nFeel free to add:\n\nReviewed-by: Paul Menzel \n\n\nKind regards,\n\nPaul","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","intel-wired-lan@lists.osuosl.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","intel-wired-lan@lists.osuosl.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=osuosl.org header.i=@osuosl.org header.a=rsa-sha256\n header.s=default header.b=mDEednAt;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=osuosl.org\n (client-ip=2605:bc80:3010::136; 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