Re: [PATCH v2 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest

[Peter Zijlstra]

On Mon, Nov 09, 2020 at 10:12:41AM +0800, Like Xu wrote:
> With PEBS virtualization, the PEBS records get delivered to the guest,
> and host still sees the PEBS overflow PMI from guest PEBS counters.
> This would normally result in a spurious host PMI and we needs to inject
> that PEBS overflow PMI into the guest, so that the guest PMI handler
> can handle the PEBS records.
> 
> Check for this case in the host perf PEBS handler. If a PEBS overflow
> PMI occurs and it's not generated from host side (via check host DS),
> a fake event will be triggered. The fake event causes the KVM PMI callback
> to be called, thereby injecting the PEBS overflow PMI into the guest.
> 
> No matter how many guest PEBS counters are overflowed, only triggering
> one fake event is enough. The guest PEBS handler would retrieve the
> correct information from its own PEBS records buffer.
> 
> If the counter_freezing is disabled on the host, a guest PEBS overflow
> PMI would be missed when a PEBS counter is enabled on the host side
> and coincidentally a host PEBS overflow PMI based on host DS_AREA is
> also triggered right after vm-exit due to the guest PEBS overflow PMI
> based on guest DS_AREA. In that case, KVM will disable guest PEBS before
> vm-entry once there's a host PEBS counter enabled on the same CPU.

How does this guest DS crud work? DS_AREA is a host virtual address;
ISTR there was lots of fail trying to virtualize it earlier. What's
changed? There's 0 clues here.

Why are the host and guest DS area separate, why can't we map them to
the exact same physical pages?