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

[Maxim Levitsky <mlevitsk@redhat.com>]

On Mon, 2019-06-03 at 18:25 -0400, John Snow wrote:
> 
> On 4/17/19 3:53 PM, Maxim Levitsky wrote:
> > Phase bits are only set by the hardware to indicate new completions
> > and not by the device driver.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > ---
> >  block/nvme.c | 2 --
> >  1 file changed, 2 deletions(-)
> > 
> > diff --git a/block/nvme.c b/block/nvme.c
> > index 0684bbd077..2d208000df 100644
> > --- a/block/nvme.c
> > +++ b/block/nvme.c
> > @@ -340,8 +340,6 @@ static bool nvme_process_completion(BDRVNVMeState *s, NVMeQueuePair *q)
> >          qemu_mutex_lock(&q->lock);
> >          c->cid = cpu_to_le16(0);
> >          q->inflight--;
> > -        /* Flip Phase Tag bit. */
> > -        c->status = cpu_to_le16(le16_to_cpu(c->status) ^ 0x1);
> >          progress = true;
> >      }
> >      if (progress) {
> > 
> 
> Since you've not got much traction on this and you've pinged a v2, can
> you point me to a spec or a reproducer that illustrates the problem?
> 
> (Or wait for more NVME knowledgeable people to give you a review...!)

"A Completion Queue entry is posted to the Completion Queue when the controller write of that Completion
Queue entry to the next free Completion Queue slot inverts the Phase Tag (P) bit from its previous value
in memory. The controller may generate an interrupt to the host to indicate that one or more Completion
Queue entries have been posted."



Best regards,
	Maxim Levitsky