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

[Stefan Hajnoczi <stefanha@gmail.com>]

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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),

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

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