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

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

On 08.10.2019 16:24, 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.


Feel better!


>
> 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.)


Why don't we just migrate host_features? That will ensure they are the 
same. And messing up device config on destination is bad anyway.


>
> Stefan