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

[Evgeny Yakovlev <wrfsh@yandex-team.ru>]

On 09.10.2019 22:14, Michael S. Tsirkin wrote:
> 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
>
Thanks, will do as you guys both suggested.