Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion

[Rik van Riel]

On Tue, 2017-07-25 at 07:46 -0700, Dan Williams wrote:
> On Tue, Jul 25, 2017 at 7:27 AM, Pankaj Gupta <address@hidden>
> wrote:
> > 
> > Looks like only way to send flush(blk dev) from guest to host with
> > nvdimm
> > is using flush hint addresses. Is this the correct interface I am
> > looking?
> > 
> > blkdev_issue_flush
> >  submit_bio_wait
> >   submit_bio
> >     generic_make_request
> >       pmem_make_request
> >       ...
> >            if (bio->bi_opf & REQ_FLUSH)
> >                 nvdimm_flush(nd_region);
> 
> I would inject a paravirtualized version of pmem_make_request() that
> sends an async flush operation over virtio to the host. Don't try to
> use flush hint addresses for this, they don't have the proper
> semantics. The guest should be allowed to issue the flush and receive
> the completion asynchronously rather than taking a vm exist and
> blocking on that request.

That is my feeling, too. A slower IO device benefits
greatly from an asynchronous flush mechanism.

-- 
All rights reversed

signature.asc
Description: This is a digitally signed message part