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

On Thu, Dec 7, 2023 at 1:16 PM Jason Wang <[email protected]> wrote:
>
> On Wed, Dec 6, 2023 at 6:17 PM Michael S. Tsirkin <[email protected]> wrote:
> >
> > On Wed, Dec 06, 2023 at 05:16:04PM +0800, Jason Wang wrote:
> > > On Tue, Dec 5, 2023 at 6:58 PM David Stevens <[email protected]> wrote:
> > > >
> > > > On Tue, Dec 5, 2023 at 1:18 PM Jason Wang <[email protected]> wrote:
> > > > >
> > > > > On Mon, Dec 4, 2023 at 5:41 PM David Stevens <[email protected]> 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/[email protected]/
> > >
> > > 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?
>
> It looks to me we need:
>
> 1) Device side, if device can preserve state on d3hot, it may
> advertise no_soft_reset
> 2) Driver side, if driver sees no_soft_reset, it doesn't need to
> manually suspend and resume. But in the above patch, it only checks
> the existence of PM, it looks to me it needs to check no_soft_reset
> instead.

Right, my original reply was needlessly vague. Similar to the linked
patch, but based on checking No_Soft_Reset instead of just pm_cap.

That also raises the point that the proposed spec changes are also
insufficient - the PCI section needs to say that setting No_Soft_Reset
is required for low power mode.

-David

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