Re: [virtio-dev] Re: [PATCH v1] virtio-mmio: Specify wait needed in driver during reset

[Jason Wang <jasowang@redhat.com>]

在 2021/8/16 下午7:48, Michael S. Tsirkin 写道:
> On Mon, Aug 16, 2021 at 12:05:50PM +0530, Srivatsa Vaddagiri wrote:
> > * Michael S. Tsirkin <mst@redhat.com> [2021-08-16 01:35:46]:
> >
> > > So thinking about all this, quite early in the setup process we
> > > have:
> > >
> > >          /* Figure out what features the device supports. */
> > >          device_features = dev->config->get_features(dev);
> > >
> > >
> > > isn't this sufficient? this will flush out
> > > all writes and device can defer responding to reads
> > > until reset is complete.
> > Hmm not sure if that's possible in all MMIO devices? I think a while back Jason
> > felt some of the MMIO devices, their registers could act as plain DRAM for
> > reads, which means device can't block such reads?
> >
> > - vatsa
> Is it worth bothering about theoretical issues though? Jason what is
> your take?


Re-read the spec, and it said

"

MMIO Device Register Layout

"

I believe plain DRAM will not behave like a register. So we don't need 
to worry about that.

But they are indeed devices that act as a plain DRAM for their control 
path, which might requires new transports[1].

Another question is that what makes PCI differ from MMIO. (E.g PCI 
requires a re-read but MMIO doesn't)

[1] https://lkml.org/lkml/2020/9/1/255


> > ---
> >
> > Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a
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