Re: [virtio-comment] Hardware friendly proposals from Intel for packed-ring-layout

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/08/2017 16:23, Michael S. Tsirkin wrote:
> On Thu, Aug 24, 2017 at 02:10:34PM +0200, Paolo Bonzini wrote:
>> On 24/08/2017 13:53, Tiwei Bie wrote:
>>>
>>> * In addition to the DESC_HW flag, each virtio queue has a tail pointer
>>>     - Driver creates suitable (i.e. multiple of cacheline) descriptors,
>>>       then performs MMIO write to tail pointer.
>>
>> If I understand correctly, the tail pointer is the value that is written
>> to the MMIO register.  If that is the case, this is unfortunately bad
>> for virtualization.  Virt prefers a doorbell register where the value
>> doesn't matter.
> 
> I think it's fine actually, this tail pointer merely duplicates data
> that is already available in form of DESC_HW flag.  Virt can just ignore
> the value.

If the spec says the hardware can read speculatively beyond the tail
pointer, that's fine indeed (that was not in Tiwei's proposal).

Paolo