Re: [Qemu-block] [Qemu-devel] [PATCH v2 1/5] block/nvme: don't flip CQ phase bits

[Maxim Levitsky]

On Fri, 2019-06-07 at 15:28 -0400, John Snow wrote:
> 
> On 6/7/19 7:08 AM, Paolo Bonzini wrote:
> > On 06/06/19 23:23, John Snow wrote:
> > > So: This looks right; does this fix a bug that can be observed? Do we
> > > have any regression tests for block/NVMe?
> > 
> > I don't think it fixes a bug; by the time the CQ entry is picked up by
> > QEMU, the device is not supposed to touch it anymore.
> > 
> > However, the idea behind the phase bits is that you can decide whether
> > the driver has placed a completion in the queue.  When we get here, we have
> > 
> >     le16_to_cpu(c->status) & 0x1) == !q->cq_phase
> > 
> > On the next pass through the ring buffer q->cq_phase will be flipped,
> > and thus when we see this element we'll get
> > 
> >     le16_to_cpu(c->status) & 0x1) == q->cq_phase
> > 
> > and not process it.  Since block/nvme.c flips the bit, this mechanism
> > does not work and the loop termination relies on the other part of the
> > condition, "if (!c->cid) break;".
> > 
> > So the patch is correct, but it would also be nice to also either remove
> > phase handling altogether, or check that the phase handling works
> > properly and drop the !c->cid test.
> > 
> > Paolo


I agree with that and I'll send an updated patch soon.

The driver should not touch the completion entries at all, but rather just scan 
for the entries whose
phase bit was flipped by the hardware.

in fact I don't even think that the 'c->cid' became the exit condition, but 
rather since the device is not allowed 
to fully fill the compleiton queue (it must alway keep at least one free entry 
there), the end condition would still
be the check on the flipped phase bit.


I'll fix that to be up to the spec,

Best regards,
        Maxim Levitskky