"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> 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
