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

[Pankaj Gupta]

> 
> 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.

Thanks for all the suggestions!

Just want to summarize here(high level):

This will require implementing new 'virtio-pmem' device which presents 
a DAX address range(like pmem) to guest with read/write(direct access)
& device flush functionality. Also, qemu should implement corresponding
support for flush using virtio.

Thanks,
Pankaj
> 
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