Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics event

[Ravi Bangoria <ravi.bangoria@amd.com>]

On 29-Sep-23 5:23 PM, Peter Zijlstra wrote:
> On Wed, Sep 27, 2023 at 10:27:07AM -0700, Sean Christopherson wrote:
> 
>> I don't think it does work, at least not without a very, very carefully crafted
>> setup and a host userspace that knows it must not use certain aspects of perf.
>> E.g. for PEBS, if the guest virtual counters don't map 1:1 to the "real" counters
>> in hardware, KVM+perf simply disables the counter.
> 
> I have distinct memories of there being patches to rewrite the PEBS
> buffer, but I really can't remember what we ended up doing. Like I said,
> I can't operate KVM in any meaningful way -- it's a monster :-(
> 
>> And for top-down slots, getting anything remotely accurate requires pinning vCPUs
>> 1:1 with pCPUs and enumerating an accurate toplogy to the guest:
>>
>>   The count is distributed among unhalted logical processors (hyper-threads) who
>>   share the same physical core, in processors that support Intel Hyper-Threading
>>   Technology.
> 
> So IIRC slots is per logical CPU, it counts the actual pipeline stages
> going towards that logical CPU, this is required to make it work on SMT
> at all -- even for native.
> 
> But it's been a long while since that was explained -- and because it
> was a call, I can't very well read it back, god how I hate calls :-(
> 
>> Jumping the gun a bit (we're in the *super* early stages of scraping together a
>> rough PoC), but I think we should effectively put KVM's current vPMU support into
>> maintenance-only mode, i.e. stop adding new features unless they are *very* simple
>> to enable, and instead pursue an implementation that (a) lets userspace (and/or
>> the kernel builder) completely disable host perf (or possibly just host perf usage
>> of the hardware PMU) and (b) let KVM passthrough the entire hardware PMU when it
>> has been turned off in the host.
> 
> I don't think you need to go that far, host can use PMU just fine as
> long as it doesn't overlap with a vCPU. Basically, if you force
> perf_attr::exclude_guest on everything your vCPU can haz the full thing.
> 
>> Hardware vendors are pushing us in the direction whether we like it or not, e.g.
>> SNP and TDX want to disallow profiling the guest from the host, 
> 
> Yeah, sekjoerity model etc.. bah.
> 
>> ARM has an upcoming PMU model where (IIUC) it can't be virtualized
>> without a passthrough approach,
> 
> :-(
> 
>> Intel's hybrid CPUs are a complete trainwreck unless vCPUs are pinned,
> 
> Anybodies hybrid things are a clusterfuck, hybrid vs virt doesn't work
> sanely on ARM either AFAIU.
> 
> I intensely dislike hybrid (and virt ofc), but alas we get to live with
> that mess :/ And it's only going to get worse I fear..
> 
> At least (for now) AMD hybrid is committed to identical ISA, including
> PMUs with their Zen4+Zen4c things. We'll have to wait and see how
> that'll end up.
> 
>> and virtualizing things like top-down slots, PEBS, and LBRs in the shared model
>> requires an absurd amount of complexity throughout the kernel and userspace.
> 
> I'm not sure about top-down, the other two, for sure.
> 
> My main beef with top-down is the ludicrously bad hardware interface we
> have on big cores, I like the atom interface a *ton* better.
> 
>> Note, a similar idea was floated and rejected in the past[*], but that failed
>> proposal tried to retain host perf+PMU functionality by making the behavior dynamic,
>> which I agree would create an awful ABI for the host.  If we make the "knob" a
>> Kconfig 
> 
> Must not be Kconfig, distros would have no sane choice.
> 
>> or kernel param, i.e. require the platform owner to opt-out of using perf
>> no later than at boot time, then I think we can provide a sane ABI, keep the
>> implementation simple, all without breaking existing users that utilize perf in
>> the host to profile guests.
> 
> It's a shit choice to have to make. At the same time I'm not sure I have
> a better proposal.

How about keying off based on PMU specific KVM module parameter? Something
like what Manali has proposed for AMD VIBS? Please see solution 1.1:

https://lore.kernel.org/r/3a6c693e-1ef4-6542-bc90-d4468773b97d@amd.com

> It does mean a host cannot profile one guest and have pass-through on the
> other. Eg. have a development and production guest on the same box. This
> is pretty crap.
> 
> Making it a guest-boot-option would allow that, but then the host gets
> complicated again. I think I can make it trivially work for per-task
> events, simply error the creation of events without exclude_guest for
> affected vCPU tasks. But the CPU events are tricky.
> 
> 
> I will firmly reject anything that takes the PMU away from the host
> entirely through.
> 
> Also, NMI watchdog needs a solution.. Ideally hardware grows a second
> per-CPU timer we can program to NMI.

Thanks,
Ravi