Re: [PATCH 2/4] arm64/x86: KVM: Introduce steal time cap

[Marc Zyngier <maz@kernel.org>]

On 2020-06-22 11:31, Andrew Jones wrote:
> On Mon, Jun 22, 2020 at 10:51:47AM +0100, Marc Zyngier wrote:
>> On 2020-06-22 09:41, Andrew Jones wrote:
>> > On Mon, Jun 22, 2020 at 09:20:02AM +0100, Marc Zyngier wrote:
>> > > Hi Andrew,
>> > >
>> > > On 2020-06-19 19:46, Andrew Jones wrote:
>> > > > arm64 requires a vcpu fd (KVM_HAS_DEVICE_ATTR vcpu ioctl) to probe
>> > > > support for steal time. However this is unnecessary and complicates
>> > > > userspace (userspace may prefer delaying vcpu creation until after
>> > > > feature probing). Since probing steal time only requires a KVM fd,
>> > > > we introduce a cap that can be checked.
>> > >
>> > > So this is purely an API convenience, right? You want a way to
>> > > identify the presence of steal time accounting without having to
>> > > create a vcpu? It would have been nice to have this requirement
>> > > before we merged this code :-(.
>> >
>> > Yes. I wish I had considered it more closely when I was reviewing the
>> > patches. And, I believe we have yet another user interface issue that
>> > I'm looking at now. Without the VCPU feature bit I'm not sure how easy
>> > it will be for a migration to fail when attempting to migrate from a
>> > host
>> > with steal-time enabled to one that does not support steal-time. So it's
>> > starting to look like steal-time should have followed the pmu pattern
>> > completely, not just the vcpu device ioctl part.
>> 
>> Should we consider disabling steal time altogether until this is 
>> worked out?
> 
> I think we can leave it alone and just try to resolve it before merging
> QEMU patches (which I'm working on now). It doesn't look like kvmtool 
> or
> rust-vmm (the only other two KVM userspaces I'm paying some attention 
> to)
> do anything with steal-time yet, so they won't notice. And, I'm not 
> sure
> disabling steal-time for any other userspaces is better than just 
> trying
> to keep them working the best we can while improving the uapi.

Is it only migration that is affected? Or do you see issues that would
affect non-migrating userspace?

[...]

>> Accepting the pvtime attributes (setting up the per-vcpu area) has two
>> effects: we promise both the guest and userspace that we will provide
>> the guest with steal time. By not checking sched_info_on(), we lie to
>> both, with potential consequences. It really feels like a bug.
> 
> Yes, I agree now. Again, following the pmu pattern looks best here. The
> pmu will report that it doesn't have the attr support when its 
> underlying
> kernel support (perf counters) doesn't exist. That's a direct analogy 
> with
> steal-time relying on sched_info_on().

Indeed. I'd be happy to take a fix early if you can spin one.

> I'll work up another version of this series doing that, but before 
> posting
> I'll look at the migration issue a bit more and likely post something 
> for
> that as well.

OK. I'll park this series for now.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...
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