Re: [virtio-dev] Re: [PATCH v2] virtio-pmem: PMEM device spec

[Pankaj Gupta <pankaj.gupta.linux@gmail.com>]

On Thu, 19 Aug 2021 at 11:39, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Thu, Aug 19, 2021 at 07:53:48AM +0200, Pankaj Gupta wrote:
> > > >> > +\drivernormative{\subsubsection}{Driver Initialization: Virtio flush}{Device Types / PMEM Driver / Driver Initialization / Virtio flush}
> > > >> > +
> > > >> > +The driver MUST implement a virtio based flushing interface.
> > > >>
> > > >> I think this can be dropped. The only way to make writes persistent is
> > > >> by sending flush requests. There is no need to say that the driver has
> > > >> to use this interface. It doesn't really have to, but then it won't be
> > > >> able to guarantee persistence.
> > > >
> > > > I am not 100 percent sure on this.
> > >
> > > I don't think we want this in a normative section, but maybe there's a
> > > place for it in a section that describes the general operation?
> >
> > I was saying because this is the must property for virtio-pmem device
> > which is emulated
> > on regular block device. virtio-pmem above real persistent devices
> > don't need any explicit flush.
> >
> > This can be enhanced later for real persistent devices which don't
> > need explicit flush
> > and thus driver writers don't have to worry about it.
>
> A feature bit can be added in the future that hints to the driver that
> flushes are not necessary (only CPU cache flushes are required for
> persistence).

Sure. will remove this then.
>
> However, I remember seeing specifications for persistent memory devices
> that do require explicit flush operations (not just a CPU cache flush).
> Let's keep them in mind too.

Thanks. Maybe internally they would be doing CPU flushes (a guess) and/or
some metadata sync operation.

Thank you for all the feedback. Will send a v3 with the suggested changes.

Best regards,
Pankaj

>
> Stefan