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

[Mingwei Zhang]

On Tue, Oct 17, 2023 at 3:24 AM Manali Shukla <manali.shukla@xxxxxxx> wrote:
>
> On 10/11/2023 7:45 PM, Peter Zijlstra wrote:
> > On Mon, Oct 09, 2023 at 10:33:41PM +0530, Manali Shukla wrote:
> >> Hi all,
> >>
> >> I would like to add following things to the discussion just for the awareness of
> >> everyone.
> >>
> >> Fully virtualized PMC support is coming to an upcoming AMD SoC and we are
> >> working on prototyping it.
> >>
> >> As part of virtualized PMC design, the PERF_CTL registers are defined as Swap
> >> type C: guest PMC states are loaded at VMRUN automatically but host PMC states
> >> are not saved by hardware.
> >
> > Per the previous discussion, doing this while host has active counters
> > that do not have ::exclude_guest=1 is invalid and must result in an
> > error.
> >
>
> Yeah, exclude_guest should be enforced on host, while host has active PMC
> counters and VPMC is enabled.
>
> > Also, I'm assuming it is all optional, a host can still profile a guest
> > if all is configured just so?
> >
>
> Correct, host should be able to profile guest, if VPMC is not enabled.
>
> >> If hypervisor is using the performance counters, it
> >> is hypervisor's responsibility to save PERF_CTL registers to host save area
> >> prior to VMRUN and restore them after VMEXIT.
> >
> > Does VMEXIT clear global_ctrl at least?
> >
>
> global_ctrl will be initialized to reset value(0x3F) during VMEXIT. Similarly,
> all the perf_ctl and perf_ctr are initialized to reset values(0) at VMEXIT.

Are these guest values (automatically) saved in the guest area of VMCB
on VMEXIT?

>
> >> In order to tackle PMC overflow
> >> interrupts in guest itself, NMI virtualization or AVIC can be used, so that
> >> interrupt on PMC overflow in guest will not leak to host.
> >
> > Can you please clarify -- AMD has this history with very dodgy PMI
> > boundaries. See the whole amd_pmu_adjust_nmi_window() crud. Even the
> > PMUv2 update didn't fix that nonsense.
> >
> > How is any virt stuff supposed to fix this? If the hardware is late
> > delivering PMI, what guarantees a guest PMI does not land in host
> > context and vice-versa?
> >
> > How does NMI virtualization (what even is that) or AVIC (I'm assuming
> > that's a virtual interrupt controller) help?
> >
>
> When NMI virtualization is enabled and source of VNMI is in guest, micro code will
> make sure that VNMI will directly be delivered to the guest (even if NMI was late
> delivered), so there is no issue of leaking guest NMI to the host.
>
> > Please make very sure, with your hardware team, that PMI must not be
> > delivered after clearing global_ctrl (preferably) or at the very least,
> > there exists a sequence of operations that provides a hard barrier
> > to order PMI.
> >
> We are verifying all the corner cases, while prototyping PMC virtualization.
> As of now, we don't see guest NMIs leaking to host issue. But latency issues
> still stays.

How long is the latency? From the existing code (amd_pmu_disable_all()
=> amd_pmu_check_overflow()), it seems to take up to 50 microseconds
in amd_pmu_check_overflow(). But I wonder how much in reality.

Thanks.
-Mingwei
>