Re: [virtio-comment] [RFC] Define a low power state for devices

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

On Thu, Apr 13, 2023 at 03:29:43PM +0900, David Stevens wrote:
> On Fri, Apr 7, 2023 at 8:27 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Apr 06, 2023 at 04:30:02PM +0900, David Stevens wrote:
> > > This RFC defines a low power state for virtio devices, to gives
> > > drivers an option for power management besides simply resetting their
> > > device.
> > >
> > > This patch is a draft aimed at starting a discussion, rather than being
> > > a finalized patch.
> > >
> > > To provide some context on where this is coming from, I'm working on
> > > reducing the power overhead of ARCVM (virtualized Android running on
> > > ChromeOS). One of the big gaps in ARCVM power management is that it does
> > > not implement Android's partial wake locks - i.e. the (virtualized) CPUs
> > > are always on, even if the (virtualized) screen is off. This means we
> > > can't force apps to stop running when they shouldn't be running, which
> > > can lead to higher power consumption compared to the ChromeOS baseline.
> > >
> > > Partial wake locks are built on s2idle, but unfortunately the current
> > > power management of virtio drivers does not let us use s2idle. The fact
> > > that power management is based around resetting the virtio device means
> > > that it doesn't work with stateful devices (e.g. virtio-fs). Even for
> > > stateless devices, re-initializing all of the devices takes longer than
> > > we would like, especially on lower end hardware.
> > >
> > > My rough idea for how to address this would be to make the existing
> > > virtio power management targeted at S4 specifically (i.e. the freeze
> > > device callback). For S0/S1/S3 (i.e. the suspend device callback), this
> > > new lighter weight low power state would be used if available -
> > > otherwise it would fall back to the existing S4 power management code.
> > >
> > > I have a very rough prototype that seems to work, and I haven't seen
> > > anything that makes me think this approach is fundamentally unworkable.
> > > That said, I would like to get feedback on the approach earlier rather
> > > than later.
> > > ---
> > >  content.tex | 26 ++++++++++++++++++++++++++
> > >  1 file changed, 26 insertions(+)
> > >
> > > diff --git a/content.tex b/content.tex
> > > index cff548ab9675..01da6f62ef20 100644
> > > --- a/content.tex
> > > +++ b/content.tex
> > > @@ -449,6 +449,28 @@ \section{Exporting Objects}\label{sec:Basic Facilities of a Virtio Device / Expo
> > >  types. It is RECOMMENDED that devices generate version 4
> > >  UUIDs as specified by \hyperref[intro:rfc4122]{[RFC4122]}.
> > >
> > > +\section{Low Power Mode}\label{sec:Basic Facilities of a Virtio Device / Low Power Mode}
> > > +
> > > +A virtio device can be put into a low power state when the
> > > +VIRTIO_F_LOW_POWER bit is negotiated. How a driver puts a
> > > +device into a low power state is transport specific.
> > > +
> > > +In general, a virtio device in a low power state should
> > > +avoid initating any communication with the driver. However,
> > > +certain device-specific functionality is exempt from this
> > > +requirement. Such functionality is detailed in the device
> > > +type specifications.
> > > +
> > > +% One example of such functionality would be allowing
> > > +% the virtio-net device to wake up the guest to deliver
> > > +% incoming network packets.
> > > +
> > > +While a virtio device is in a low power state, it should
> > > +maintain any type specific state in such a way that it is
> > > +able to immediately resume functioning upon leaving the low
> > > +power state, without the need for any additional
> > > +communication with the driver.
> > > +
> > >  \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}
> > >
> > >  We start with an overview of device initialization, then expand on the
> > > @@ -803,6 +825,10 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
> > >    that the driver can reset a queue individually.
> > >    See \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}.
> > >
> > > +  \item[VIRTIO_F_LOW_POWER(41)] This feature indicates
> > > +  that the driver can put the device into a low power mode.
> > > +  See \ref{sec:Basic Facilities of a Virtio Device / Low Power Mode}.
> > > +
> > >  \end{description}
> > >
> > >  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> >
> > So what purpose does this flag serve exactly?  I guess transports also
> > provide ways to enumerate supported power states, no?
> 
> This is mostly here to parallel the VIRTIO_F_SR_IOV feature flag.
> Generally speaking, it does seem redundant with the transport-specific
> enumeration.
> 
> The two potential uses I can think of would be to allow devices to
> support transport level power management without supporting virtio
> level power management (might apply to existing devices?) and to allow
> devices to behave differently if they know that the driver doesn't
> support virtio power management. But I don't know how useful these are
> in practice.
> 
> -David

I'm a bit confused by all this. So there are actually two types of PM?
What does initiating communication involve? Is consuming buffers
initiating communication? Sending interrupts?


-- 
MST


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