On Mon, Jul 08, 2013 at 10:51:39AM +0930, Rusty Russell wrote: > "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: > > On Thu, Jul 04, 2013 at 10:49:42AM +0300, Michael S. Tsirkin wrote: > >> > case). > >> > > >> > Regrettably, initial driver implementations used simple layouts > >> > and devices came to rely on it, despite this specification > >> > wording. It is thus recommended that drivers be conservative in > >> > their assumptions, unless specific device features indicate that > >> > general layout is permitted using VIRTIO_F_ANY_LAYOUT. In > >> > addition, some implementations may have large-but-reasonable > >> > restrictions on total descriptor size (such as based on IOV_MAX > >> > in the host OS). This has not been a problem in practice: little > >> > sympathy will be given to drivers which create unreasonably-sized > >> > descriptors such as dividing a network packet into 1500 > >> > single-byte descriptors! > >> > >> That's fine with me too. > >> So which bit are we using for this? > >> I'd like to rebase to latest bits and merge the optimization for 3.11. > > > > > > Rusty, could you please tell me which feature bit do you > > prefer for ANY_LAYOUT? > > It would be sad to miss another release of both qemu and kernel with > > this obvious optimization for the only reason we can't settle on a bit > > to use to signal it. > > Let's use bit 30. Here's the kernel patch: > > virtio: VIRTIO_F_ANY_LAYOUT feature > > Also known as the "no really, I read the spec" bit. > > Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx> This is already used in qemu: /* A guest should never accept this. It implies negotiation is broken. */ #define VIRTIO_F_BAD_FEATURE 30 > diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h > index b7cda39..4b5da48 100644 > --- a/include/uapi/linux/virtio_config.h > +++ b/include/uapi/linux/virtio_config.h > @@ -51,4 +51,7 @@ > * suppressed them? */ > #define VIRTIO_F_NOTIFY_ON_EMPTY 24 > > +/* Can the device handle any descriptor layout? */ > +#define VIRTIO_F_ANY_LAYOUT 30 > + > #endif /* _UAPI_LINUX_VIRTIO_CONFIG_H */ _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
