Re: [PATCH net] vsock/virtio: fix zerocopy completion for multi-skb sends

[Stefano Garzarella]

On Tue, May 19, 2026 at 09:37:23AM +0300, Arseniy Krasnov wrote:
> On 18/05/2026 14:08, Stefano Garzarella wrote:
> > On Mon, May 18, 2026 at 11:50:05AM +0100, David Laight wrote:
> > > On Mon, 18 May 2026 11:54:19 +0200
> > > Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:

[...]

> Hi guys! Just some replies after quick look:
>
> > > > >> > Surely that block should only be done if can_zcopy is true?
>
> I guess no, since TCP also allocates uarg even if zerocopy is impossible -
> it just sets uarg_to_msgzc(uarg)->zerocopy = 0;
>
> > > > >> > And shouldn't something unset it if info->op != VIRTIO_VSOCK_OP_RW ?
>
> Hm, we can't enter block 'if (info->msg) {' when 'info->op' is not equal to 'VIRTIO_VSOCK_OP_RW',
> because 'msg' is not NULL only for 'VIRTIO_VSOCK_OP_RW'. But anyway You point to right thing - check for
> 'VIRTIO_VSOCK_OP_RW' could be removed here. Just with comment why.
>
> > > > >> > If the msg_zerocopy_realloc() fails then can't you just set can_zcopy to false.
>
> Here I guess it is better to follow TCP way to make same behaviour, because exact
> logic for MSG_ZEROCOPY is not documented. TCP also returns error and stops tx loop.
>
> > > > >> >
> > > > >> > It info->msg->msg_buf is already set then I think you have to disable zero-copy.
> > > > >> > The caller has already requested a callback - and you can't add another.
>
> In TCP implementation if 'msg_ubuf' is set they just use it and check for zerocopy in
> the same way as 'msg_ubuf' is NULL.
>
> > > > >> >
> > > > >> > In any case by the end of this can_zcopy and have_uref are really the same flag.
> > > > >>
>
> Need to check it more. But 'can_zcopy' means that we fill frags in skb, have_uref means that we
> allocated completion (but it could be reported with not set 'SO_EE_CODE_ZEROCOPY_COPIED' if
> 'can_zcopy' was false).
>
>
> @Stefano, I guess current implementation differs from TCP in two cases (at least from first
> view):
> 1) When 'msg_ubuf' is set: in TCP, already set 'msg_ubuf' is passed to 'skb_zerocopy_iter_stream()'
>   (if zerocopy is possible) where it is used as in vsock in call 'skb_zcopy_set()'. In vsock case if
>   'msg_ubuf' is not NULL we will pass just NULL to 'skb_zcopy_set()'. Yes this is will be
>   no-effect call today (due to checks in 'skb_zcopy_set()'), but anyway - in future may be not.
> 2) Also i see that 'skb_zerocopy_iter_stream()' in TCP version has some 
> extra checks which are
>   missed in vsock - we only just call '__zerocopy_sg_from_iter()' to fill skb in zerocopy way.
>
> But, I think, instead of trying to compare vsock and TCP versions best way is to just copy
> current TCP flow as close as possible:
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/tree/net/ipv4/tcp.c#n1144
> 1) Use same flow of checks for 'msg_flags', 'msg_ubuf', SOCK_ZEROCOPY etc.
> 2) To copy data we can use 'skb_zerocopy_iter_stream()'.
> 3) The only thing that we don't need in vsock is dev mem related code from TCP implementation.
>
> I can take this task, but pls need some time - may be two/three weeks due to another tasks.
>
> What do You think?

If you can work on it, please go head. They seems all pre-existing, so 
2/3 weeks are fine. Please let me know if you can't and I'll try to 
allocate some time.

Thanks for the help!

Stefano