Re: [PATCH v2 1/7] tap: Fix virtio-net header buffer size

[Akihiko Odaki]

On 2023/10/11 15:43, Akihiko Odaki wrote:
> On 2023/10/10 16:56, Michael Tokarev wrote:
> > 10.10.2023 05:59, Akihiko Odaki wrote:
> > > The largest possible virtio-net header is struct virtio_net_hdr_v1_hash.
> > >
> > > Fixes: fbbdbddec0 ("tap: allow extended virtio header with hash info")
> > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > ---
> > >   net/tap.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/net/tap.c b/net/tap.c
> > > index c6639d9f20..ea46feeaa8 100644
> > > --- a/net/tap.c
> > > +++ b/net/tap.c
> > > @@ -118,7 +118,7 @@ static ssize_t tap_receive_iov(NetClientState 
> > > *nc, const struct iovec *iov,
> > >       TAPState *s = DO_UPCAST(TAPState, nc, nc);
> > >       const struct iovec *iovp = iov;
> > >       struct iovec iov_copy[iovcnt + 1];
> > > -    struct virtio_net_hdr_mrg_rxbuf hdr = { };
> > > +    struct virtio_net_hdr_v1_hash hdr = { };
> >
> > BTW, can we get rid of (implicit) memzero() here and in
> > similar places, initializing only the actually used fields?
> > Not that his particular structure is very large (and this
> > change makes it 8 bytes larger), but still..
>
> These variables are rarely used so I think it's fine. In particular, 
> these variables are used only when QEMU sends an ARP packet while 
> virtio-net is enabled.

My previous statement was not correct. These variables are used also 
when QEMU emulates a network interface that doesn't expose virtio-net 
headers.

I split the code path for these two cases in v3. The header size is 
shrunken now for the normal code path.

Regards,
Akihiko Odaki