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Date:   Thu, 23 Jan 2020 08:59:06 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Haiyang Zhang <haiyangz@microsoft.com>
Cc:     sashal@kernel.org, linux-hyperv@vger.kernel.org,
        netdev@vger.kernel.org, kys@microsoft.com, sthemmin@microsoft.com,
        olaf@aepfle.de, vkuznets@redhat.com, davem@davemloft.net,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH V3,net-next, 1/2] hv_netvsc: Add XDP support
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On Wed, 22 Jan 2020 09:23:33 -0800, Haiyang Zhang wrote:
> This patch adds support of XDP in native mode for hv_netvsc driver, and
> transparently sets the XDP program on the associated VF NIC as well.
> 
> Setting / unsetting XDP program on synthetic NIC (netvsc) propagates to
> VF NIC automatically. Setting / unsetting XDP program on VF NIC directly
> is not recommended, also not propagated to synthetic NIC, and may be
> overwritten by setting of synthetic NIC.
> 
> The Azure/Hyper-V synthetic NIC receive buffer doesn't provide headroom
> for XDP. We thought about re-use the RNDIS header space, but it's too
> small. So we decided to copy the packets to a page buffer for XDP. And,
> most of our VMs on Azure have Accelerated  Network (SRIOV) enabled, so
> most of the packets run on VF NIC. The synthetic NIC is considered as a
> fallback data-path. So the data copy on netvsc won't impact performance
> significantly.
> 
> XDP program cannot run with LRO (RSC) enabled, so you need to disable LRO
> before running XDP:
>         ethtool -K eth0 lro off
> 
> XDP actions not yet supported:
>         XDP_REDIRECT
> 
> Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
> 
> ---
> Changes:
> 	v3: Minor code and comment updates.
>         v2: Added XDP_TX support. Addressed review comments.

How does the locking of the TX path work? You seem to be just calling
the normal xmit method, but you don't hold the xmit queue lock, so the
stack can start xmit concurrently, no?