Re: [PATCH v2 09/18] arm64: KVM: enable conditional save/restore full SPE profiling buffer controls

[Marc Zyngier]

On 2020-01-08 11:58, Will Deacon wrote:
> On Wed, Jan 08, 2020 at 11:17:16AM +0000, Marc Zyngier wrote:
> > On 2020-01-07 15:13, Andrew Murray wrote:
> > > On Sat, Dec 21, 2019 at 02:13:25PM +0000, Marc Zyngier wrote:
> > > > On Fri, 20 Dec 2019 14:30:16 +0000
> > > > Andrew Murray <andrew.murray@xxxxxxx> wrote:
> > > >
> > > > [somehow managed not to do a reply all, re-sending]
> > > >
> > > > > From: Sudeep Holla <sudeep.holla@xxxxxxx>
> > > > >
> > > > > Now that we can save/restore the full SPE controls, we can enable it
> > > > > if SPE is setup and ready to use in KVM. It's supported in KVM only if
> > > > > all the CPUs in the system supports SPE.
> > > > >
> > > > > However to support heterogenous systems, we need to move the check if
> > > > > host supports SPE and do a partial save/restore.
> > > >
> > > > No. Let's just not go down that path. For now, KVM on heterogeneous
> > > > systems do not get SPE.
> > >
> > > At present these patches only offer the SPE feature to VCPU's where the
> > > sanitised AA64DFR0 register indicates that all CPUs have this support
> > > (kvm_arm_support_spe_v1) at the time of setting the attribute
> > > (KVM_SET_DEVICE_ATTR).
> > >
> > > Therefore if a new CPU comes online without SPE support, and an
> > > existing VCPU is scheduled onto it, then bad things happen - which I
> > > guess
> > > must have been the intention behind this patch.
> >
> > I guess that was the intent.
> >
> > > > If SPE has been enabled on a guest and a CPU
> > > > comes up without SPE, this CPU should fail to boot (same as exposing a
> > > > feature to userspace).
> > >
> > > I'm unclear as how to prevent this. We can set the FTR_STRICT flag on
> > > the sanitised register - thus tainting the kernel if such a non-SPE CPU
> > > comes online - thought that doesn't prevent KVM from blowing up. Though
> > > I don't believe we can prevent a CPU coming up. At the moment this is
> > > my preferred approach.
> >
> > I'd be OK with this as a stop-gap measure. Do we know of any existing
> > design where only half of the CPUs have SPE?
>
> No, but given how few CPUs implement SPE I'd say that this 
> configuration
> is inevitable. I certainly went out of my way to support it in the 
> driver.
>
> > > Looking at the vcpu_load and related code, I don't see a way of saying
> > > 'don't schedule this VCPU on this CPU' or bailing in any way.
> >
> > That would actually be pretty easy to implement. In vcpu_load(), check
> > that that the CPU physical has SPE. If not, raise a request for that 
> > vcpu.
> > In the run loop, check for that request and abort if raised, returning
> > to userspace.
> >
> > Userspace can always check /sys/devices/arm_spe_0/cpumask and work out
> > where to run that particular vcpu.
>
> It's also worth considering systems where there are multiple 
> implementations
> of SPE in play. Assuming we don't want to expose this to a guest, then 
> the
> right interface here is probably for userspace to pick one SPE
> implementation and expose that to the guest. That fits with your idea 
> above,
> where you basically get an immediate exit if we try to schedule a vCPU 
> onto
> a CPU that isn't part of the SPE mask.

Then it means that the VM should be configured with a mask indicating
which CPUs it is intended to run on, and setting such a mask is 
mandatory
for SPE.

> > > One solution could be to allow scheduling onto non-SPE VCPUs but wrap
> > > the
> > > SPE save/restore code in a macro (much like kvm_arm_spe_v1_ready) that
> > > reads the non-sanitised feature register. Therefore we don't go bang,
> > > but
> > > we also increase the size of any black-holes in SPE capturing. Though
> > > this
> > > feels like something that will cause grief down the line.
> > >
> > > Is there something else that can be done?
> >
> > How does userspace deal with this? When SPE is only available on half 
> > of
> > the CPUs, how does perf work in these conditions?
>
> Not sure about userspace, but the kernel driver works by instantiating 
> an
> SPE PMU instance only for the CPUs that have it and then that instance
> profiles for only those CPUs. You also need to do something similar if
> you had two CPU types with SPE, since the SPE configuration is likely 
> to be
> different between them.

So that's closer to what Andrew was suggesting above (running a guest on 
a
non-SPE CPU creates a profiling black hole). Except that we can't really
run a SPE-enabled guest on a non-SPE CPU, as the SPE sysregs will UNDEF
at EL1.

Conclusion: we need a mix of a cpumask to indicate which CPUs we want to
run on (generic, not-SPE related), and a check for SPE-capable CPUs.
If any of these condition is not satisfied, the vcpu exits for userspace
to sort out the affinity.

I hate heterogeneous systems.

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