netdev - Re: [PATCH net-next 2/2] vhost-net: reduce one userspace copy when building XDP buff

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Message-ID: <6ebf5a0a-f13a-43c4-8e42-ba4743e7e417@redhat.com>
Date: Tue, 24 Jun 2025 16:05:38 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Jason Wang <jasowang@...hat.com>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: mst@...hat.com, eperezma@...hat.com, kvm@...r.kernel.org,
 virtualization@...ts.linux.dev, netdev@...r.kernel.org, davem@...emloft.net,
 andrew+netdev@...n.ch, edumazet@...gle.com, kuba@...nel.org
Subject: Re: [PATCH net-next 2/2] vhost-net: reduce one userspace copy when
 building XDP buff

On 6/16/25 5:01 AM, Jason Wang wrote:
> On Fri, Jun 13, 2025 at 10:16 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
>>
>> Jason Wang wrote:
>>> We used to do twice copy_from_iter() to copy virtio-net and packet
>>> separately. This introduce overheads for userspace access hardening as
>>> well as SMAP (for x86 it's stac/clac). So this patch tries to use one
>>> copy_from_iter() to copy them once and move the virtio-net header
>>> afterwards to reduce overheads.
>>>
>>> Testpmd + vhost_net shows 10% improvement from 5.45Mpps to 6.0Mpps.
>>>
>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>
>> Acked-by: Willem de Bruijn <willemb@...gle.com>
>>
>>> ---
>>>  drivers/vhost/net.c | 13 ++++---------
>>>  1 file changed, 4 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>> index 777eb6193985..2845e0a473ea 100644
>>> --- a/drivers/vhost/net.c
>>> +++ b/drivers/vhost/net.c
>>> @@ -690,13 +690,13 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
>>>       if (unlikely(!buf))
>>>               return -ENOMEM;
>>>
>>> -     copied = copy_from_iter(buf, sock_hlen, from);
>>> -     if (copied != sock_hlen) {
>>> +     copied = copy_from_iter(buf + pad - sock_hlen, len, from);
>>> +     if (copied != len) {
>>>               ret = -EFAULT;
>>>               goto err;
>>>       }
>>>
>>> -     gso = buf;
>>> +     gso = buf + pad - sock_hlen;
>>>
>>>       if (!sock_hlen)
>>>               memset(buf, 0, pad);
>>> @@ -715,12 +715,7 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
>>>               }
>>>       }
>>>
>>> -     len -= sock_hlen;
>>> -     copied = copy_from_iter(buf + pad, len, from);
>>> -     if (copied != len) {
>>> -             ret = -EFAULT;
>>> -             goto err;
>>> -     }
>>> +     memcpy(buf, buf + pad - sock_hlen, sock_hlen);
>>
>> It's not trivial to see that the dst and src do not overlap, and does
>> does not need memmove.
>>
>> Minimal pad that I can find is 32B and and maximal sock_hlen is 12B.
>>
>> So this is safe. But not obviously so. Unfortunately, these offsets
>> are not all known at compile time, so a BUILD_BUG_ON is not possible.
> 
> We had this:
> 
> int pad = SKB_DATA_ALIGN(VHOST_NET_RX_PAD + headroom + nvq->sock_hlen);
> int sock_hlen = nvq->sock_hlen;
> 
> So pad - sock_len is guaranteed to be greater than zero.
> 
> If this is not obvious, I can add a comment in the next version.

The relevant initializations are not visible in the patch itself, so I
think either a comment in the code or in the commit message would be useful.

Thanks,

Paolo