Subject: Re: [PATCH 3/3] vhost: use checked versions of VIRTIO_BIT

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 9 Oct 2025 08:54:15 -0400

On Thu, Oct 09, 2025 at 02:47:53PM +0200, Andrew Lunn wrote:
> On Thu, Oct 09, 2025 at 07:24:16AM -0400, Michael S. Tsirkin wrote:
> > This adds compile-time checked versions of VIRTIO_BIT that set bits in
> > low and high qword, respectively.  Will prevent confusion when people
> > set bits in the wrong qword.
> > 
> > Cc: "Paolo Abeni" <[email-masked]>
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> >  drivers/vhost/net.c             | 4 ++--
> >  include/linux/virtio_features.h | 9 +++++++++
> >  2 files changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 43d51fb1f8ea..8b98e1a8baaa 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -76,8 +76,8 @@ static const u64 vhost_net_features[VIRTIO_FEATURES_QWORDS] = {
> >  	(1ULL << VIRTIO_F_ACCESS_PLATFORM) |
> >  	(1ULL << VIRTIO_F_RING_RESET) |
> >  	(1ULL << VIRTIO_F_IN_ORDER),
> > -	VIRTIO_BIT(VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO) |
> > -	VIRTIO_BIT(VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO),
> > +	VIRTIO_BIT_HI(VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO) |
> > +	VIRTIO_BIT_HI(VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO),
> 
> How any bits in vhost_net_features are currently in use?

68

> How likely is
> it to go from 2x 64bit words to 3x 64 bit words?

Maybe.

> Rather than _LO, _HI,
> would _1ST, _2ND be better leaving it open for _3RD?

I can just open-code

	VIRTIO_BIT_QWORD(VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO, 1)

> I would also be tempted to rename these macros to include _LO_ and
> _HI_ in them. VIRTIO_BIT_HI(VIRTIO_LO_F_IN_ORDER) is more likely to be
> spotted as wrong this way.

Hmm but with my macros compiler will warn so why uglify then?

> An alternative would be to convert to a linux bitmap, which is
> arbitrary length so you just use bit number and leave the
> implementation to map that to the correct offset in the underlying
> data structure.
> 
> 	Andrew

Right but it's a bit more work as we then have to change
all drivers. Not ruling this out, but this patchset
is not aiming that high.

-- 
MST