linux-kernel - Re: [PATCH net 4/4] net: hns3: default enable tx bounce buffer when smmu enabled

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Message-ID: <20250704163149.GJ41770@horms.kernel.org>
Date: Fri, 4 Jul 2025 17:31:49 +0100
From: Simon Horman <horms@...nel.org>
To: Jijie Shao <shaojijie@...wei.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, andrew+netdev@...n.ch, shenjian15@...wei.com,
	liuyonglong@...wei.com, chenhao418@...wei.com,
	jonathan.cameron@...wei.com, shameerali.kolothum.thodi@...wei.com,
	salil.mehta@...wei.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 4/4] net: hns3: default enable tx bounce buffer when
 smmu enabled

On Wed, Jul 02, 2025 at 09:09:01PM +0800, Jijie Shao wrote:
> The SMMU engine on HIP09 chip has a hardware issue.
> SMMU pagetable prefetch features may prefetch and use a invalid PTE
> even the PTE is valid at that time. This will cause the device trigger
> fake pagefaults. The solution is to avoid prefetching by adding a
> SYNC command when smmu mapping a iova. But the performance of nic has a
> sharp drop. Then we do this workaround, always enable tx bounce buffer,
> avoid mapping/unmapping on TX path.
> 
> This issue only affects HNS3, so we always enable
> tx bounce buffer when smmu enabled to improve performance.
> 
> Fixes: 295ba232a8c3 ("net: hns3: add device version to replace pci revision")

The cited commit may be a pre-requisite for this patch
to check HNAE3_DEVICE_VERSION_V3. But it seems to me that the problem
being addressed existed before the cited commit. If so, I think a different
Fixes tag is appropriate.

> Signed-off-by: Jian Shen <shenjian15@...wei.com>
> Signed-off-by: Jijie Shao <shaojijie@...wei.com>
> ---
>  .../net/ethernet/hisilicon/hns3/hns3_enet.c   | 31 +++++++++++++++++
>  .../net/ethernet/hisilicon/hns3/hns3_enet.h   |  2 ++
>  .../ethernet/hisilicon/hns3/hns3_ethtool.c    | 33 +++++++++++++++++++

It seems to me that the hns3_ethtool.c changes a) are not a requirement
for the work-around introduced by this patch and b) may introduce
complex behaviour between the effect of ethtool copybreak settings
and the enablement/non-enablement of the work-around.

Would it be possible to make a more minimal fix for net, that
omits the ethtool changes. And then follow-up with them for net-next
once the fix is present there?

-- 
pw-bot: cr