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Date:   Tue, 17 Nov 2020 15:35:29 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Like Xu <like.xu@linux.intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Kan Liang <kan.liang@linux.intel.com>, luwei.kang@intel.com,
        Thomas Gleixner <tglx@linutronix.de>, wei.w.wang@intel.com,
        Tony Luck <tony.luck@intel.com>,
        Stephane Eranian <eranian@google.com>,
        Mark Gross <mgross@linux.intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 04/17] perf: x86/ds: Handle guest PEBS overflow PMI
 and inject it to guest
Message-ID: <20201117143529.GJ3121406@hirez.programming.kicks-ass.net>
References: <20201109021254.79755-1-like.xu@linux.intel.com>
 <20201109021254.79755-5-like.xu@linux.intel.com>
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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?