Re: [virtio-comment] [PATCH v3 1/1] Define a low power mode for devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Dec 06, 2023 at 05:16:04PM +0800, Jason Wang wrote:
> On Tue, Dec 5, 2023 at 6:58 PM David Stevens <stevensd@chromium.org> wrote:
> >
> > On Tue, Dec 5, 2023 at 1:18 PM Jason Wang <jasowang@redhat.com> wrote:
> > >
> > > On Mon, Dec 4, 2023 at 5:41 PM David Stevens <stevensd@chromium.org> wrote:
> > > >
> > > > Define a low power mode for virtio devices where the devices are
> > > > expected to maintain their state. This gives drivers an option for power
> > > > management besides simply resetting their device. In the virtualization
> > > > use case, this allows the guest to be suspended even with stateful
> > > > virtio devices like gpu and fs.
> > > >
> > > > Low power mode is primarily defined at the transport layer. The only
> > > > part that depends on device-type specific details is whether a given
> > > > virtqueue is device driven or driver driven.
> > > >
> > > > This change only defines the transport-specific implementation for
> > > > Virtio over PCI.
> > >
> > > A dumb question, if this is only for PCI, can the device just
> > > implement no_soft_reset via PMC?
> >
> > This is basically No_Soft_Reset, yes. If a change similar to [1] would
> > be acceptable based only on the No_Soft_Reset bit even with no concept
> > of power management in the virtio spec, then I personally don't have
> > any problems with that.
> >
> > [1] https://lore.kernel.org/lkml/20231113055138.117392-1-stevensd@chromium.org/
> 
> So if I read the code correctly, the current Qemu advertises PM but
> without no_soft_reset.
> 
> So this patch seems to break e.g virtio-net and doesn't fix virtio-GPU.
> 
> Thanks
> 
> >
> > -David
> >

what is the breakage exactly? Maybe we need a new feature bit?

-- 
MST


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