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

[Peter Zijlstra]

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.

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.