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Message-ID: <20201130104935.GN3040@hirez.programming.kicks-ass.net>
Date: Mon, 30 Nov 2020 11:49:35 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Xu, Like" <like.xu@...el.com>
Cc: Like Xu <like.xu@...ux.intel.com>,
"Kleen, Andi" <andi.kleen@...el.com>,
Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
Sean Christopherson <sean.j.christopherson@...el.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Kan Liang <kan.liang@...ux.intel.com>, luwei.kang@...el.com,
Thomas Gleixner <tglx@...utronix.de>, wei.w.wang@...el.com,
Tony Luck <tony.luck@...el.com>,
Stephane Eranian <eranian@...gle.com>,
Mark Gross <mgross@...ux.intel.com>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 04/17] perf: x86/ds: Handle guest PEBS overflow PMI
and inject it to guest
On Fri, Nov 27, 2020 at 10:14:49AM +0800, Xu, Like wrote:
> > OK, but the code here wanted to inspect the guest DS from the host. It
> > states this is somehow complicated/expensive. But surely we can at the
> > very least map the first guest DS page somewhere so we can at least
> > access the control bits without too much magic.
> We note that the SDM has a contiguous present memory mapping
> assumption about the DS save area and the PEBS buffer area.
>
> Therefore, we revisit your suggestion here and move it a bit forward:
>
> When the PEBS is enabled, KVM will cache the following values:
> - gva ds_area (kvm msr trap)
> - hva1 for "gva ds_area" (walk guest page table)
> - hva2 for "gva pebs_buffer_base" via hva1 (walk guest page table)
What this [gh]va? Guest/Host Virtual Address? I think you're assuming I
know about all this virt crap,.. I don't.
> if the "gva ds_area" cache hits,
what?
> - access PEBS "interrupt threshold" and "Counter Reset[]" via hva1
> - get "gva2 pebs_buffer_base" via __copy_from_user(hva1)
But you already had hva2, so what's the point?
> if the "gva2 pebs_buffer_base" cache hits,
What?
> - we get "gva2 pebs_index" via __copy_from_user(hva2),
pebs_index is in ds_are, which would be hva1
> - rewrite the guest PEBS records via hva2 and pebs_index
>
> If any cache misses, setup the cache values via walking tables again.
>
> I wonder if you would agree with this optimization idea,
> we look forward to your confirmation for the next step.
I'm utterly confused. I really can't follow.
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