Re: [PATCH 08/16] KVM: arm64: timers: Allow userspace to set the counter offsets

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Mar 08, 2023 at 07:46:00AM +0000, Oliver Upton wrote:
> Hey Marc,
> 
> On Thu, Feb 23, 2023 at 06:25:57PM +0000, Marc Zyngier wrote:
> 
> [...]
> 
> > > Do we need to bend over backwards for a theoretical use case with
> > > the new UAPI? If anyone depends on the existing behavior then they can
> > > continue to use the old UAPI to partially migrate the guest counters.
> > 
> > I don't buy the old/new thing. My take is that these things should be
> > cumulative if there isn't a hard reason to break the existing API.
> 
> Unsurprisingly, I may have been a bit confusing in my replies to you.
> 
> I have zero interest in breaking the existing API. Any suggestion of
> 'changing the rules' was more along the lines of providing an alternate
> scheme for the counters and letting the quirks of the old interface
> continue.
> 
> > > My previous suggestion of tying the physical and virtual counters
> > > together at VM creation would definitely break such a use case, though,
> > > so we'd be at the point of requiring explicit opt-in from userspace.
> > 
> > I'm trying to find a middle ground, so bear with me. Here's the
> > situation as I see it:
> > 
> > (1) a VM that is migrating today can only set the virtual offset and
> >     doesn't affect the physical counter. This behaviour must be
> >     preserved in we cannot prove that nobody relies on it.
> > 
> > (2) setting the physical offset could be done by two means:
> > 
> >     (a) writing the counter register (like we do for CNTVCT)
> >     (b) providing an offset via a side channel
> > 
> > I think (1) must stay forever, just like we still support the old
> > GICv2 implicit initialisation.
> 
> No argument here. Unless userspace pokes some new bit of UAPI, the old
> behavior of CNTVCT must live on.
> 
> > (2a) is also desirable as it requires no extra work on the VMM side.
> > Just restore the damn thing, and nothing changes (we're finally able
> > to migrate the physical timer). (2b) really is icing on the cake.
> > 
> > The question is whether we can come up with an API offering (2b) that
> > still allows (1) and (2a). I'd be happy with a new API that, when
> > used, resets both offsets to the same value, matching your pretty
> > picture. But the dual offsetting still has to exist internally.
> > 
> > When it comes to NV, it uses either the physical offset that has been
> > provided by writing CNTPCT, or the one that has been written via the
> > new API. Under the hood, this is the same piece of data, of course.
> > 
> > The only meaningful difference with my initial proposal is that there
> > is no new virtual offset API. It is either register writes that obey
> > the same rules as before, or a single offset setting.
> 
> I certainly agree that (2a) is highly desirable to get existing VMMs to
> 'do the right thing' for free. Playing devil's advocate, would this not
> also break the tracing example you've given of correlating timestamps
> between the host and guest? I wouldn't expect a userspace + VM tracing
> contraption to live migrate but restoring from a snapshot seems
> plausible.

The problem I'm alluding to here is that the VMM will save/restore
the physical counter value and cause KVM to offset the physical counter.
Live migration is a pretty obvious example, but resuming from a snapshot
after resetting a system be similarly affected.

> Regardless, I like the general direction you've proposed. IIUC, you'll
> want to go ahead with ignoring writes to CNT{P,V}CT if the offset was
> written by userspace, right?
> 
> -- 
> Thanks,
> Oliver
> 

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