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

[Manali Shukla]

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.

>> 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.