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

Marc Zyngier
2020-01-08T12:36:16Z

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@arm.com> wrote:
>> > >
>> > > [somehow managed not to do a reply all, re-sending]
>> > >
>> > > > From: Sudeep Holla <sudeep.holla@arm.com>
>> > > >
>> > > > 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...