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 <[email protected]> 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
