Re: [RFC PATCH] virtio-blk: advertise F_WCE (F_FLUSH) if F_CONFIG_WCE is also advertised

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 08, 2019 at 02:24:16PM +0100, Stefan Hajnoczi wrote:
> On Fri, Sep 20, 2019 at 02:56:30PM +0300, Evgeny Yakovlev wrote:
> > Virtio spec 1.1 (and earlier), 5.2.5.1 Driver Requirements: Device
> > Initialization:
> > 
> > "Devices SHOULD always offer VIRTIO_BLK_F_FLUSH, and MUST offer it if
> > they offer VIRTIO_BLK_F_CONFIG_WCE.
> > 
> > It looks like currently F_CONFIG_WCE and F_WCE are not connected to each
> > other. qemu will advertise F_CONFIG_WCE if config-wce argument is
> > set for virtio-blk device. While F_WCE is advertised if underlying block
> > backend actually has it's caching enabled.
> > Those two things are not related to each other.
> > 
> > Signed-off-by: Evgeny Yakovlev <wrfsh@yandex-team.ru>
> > ---
> >  hw/block/virtio-blk.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> > index 1885160..b45dc0c 100644
> > --- a/hw/block/virtio-blk.c
> > +++ b/hw/block/virtio-blk.c
> > @@ -990,7 +990,8 @@ static uint64_t virtio_blk_get_features(VirtIODevice *vdev, uint64_t features,
> >          virtio_add_feature(&features, VIRTIO_BLK_F_SCSI);
> >      }
> >  
> > -    if (blk_enable_write_cache(s->blk)) {
> > +    if (blk_enable_write_cache(s->blk) ||
> > +        virtio_has_feature(features, VIRTIO_BLK_F_CONFIG_WCE)) {
> >          virtio_add_feature(&features, VIRTIO_BLK_F_WCE);
> >      }
> >      if (blk_is_read_only(s->blk)) {
> > -- 
> > 2.7.4
> 
> Sorry for the very late response.  I have been ill and am still
> recovering.
> 
> The motivation for this change looks correct but this patch may cause
> host_features to change across live migration to a newer QEMU version.
> If the guest accesses VIRTIO_PCI_HOST_FEATURES before and after live
> migration it may see different values, which is unexpected.
> 
> The safe way of introducing guest-visible changes like this is to make
> the change conditional on the machine type version so that old guests
> see old behavior and new guests see new behavior.
> 
> Live migration compatibility can be guaranteed by adding a new property
> to virtio_blk_properties[]:
> 
>   DEFINE_PROP_BOOL("enable-wce-if-config-wce", VirtIOBlock,
>                    conf.enable_wce_if_config_wce, true),


is this a useful thing for users to control?
If not we don't need to make this property part of
the stable API - blacklist it by prefixing x- to the name:
x-enable-wce-if-config-wce

> Then tweak the patch:
> 
>   if (blk_enable_write_cache(s->blk) ||
>       (s->conf.enable_wce_if_config_wce &&
>        virtio_has_feature(features, VIRTIO_BLK_F_CONFIG_WCE))) {
> 
> And finally disable enable_wce_if_config_wce for older machine types to
> retain compatibility:
> 
>   GlobalProperty hw_compat_4_2[] = {
>       { "virtio-blk-device", "enable-wce-if-config-wce", "off" },
>   };
> 
> (I have omitted some steps like defining
> VirtIOBlkConf.enable_wce_if_config_wce field and hooking up
> hw_compat_4_2[], but you can figure that out from the existing code.)
> 
> Stefan