From: Wei Wang <wei.w.wang@intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
pbonzini@redhat.com, ak@linux.intel.com, kan.liang@intel.com,
mingo@redhat.com, rkrcmar@redhat.com, like.xu@intel.com,
jannh@google.com, arei.gonglei@huawei.com, jmattson@google.com
Subject: Re: [PATCH v7 10/12] KVM/x86/lbr: lazy save the guest lbr stack
Date: Tue, 09 Jul 2019 11:14:52 +0800 [thread overview]
Message-ID: <5D2406AC.20402@intel.com> (raw)
In-Reply-To: <20190708145326.GO3402@hirez.programming.kicks-ass.net>
On 07/08/2019 10:53 PM, Peter Zijlstra wrote:
> On Mon, Jul 08, 2019 at 09:23:17AM +0800, Wei Wang wrote:
>> When the vCPU is scheduled in:
>> - if the lbr feature was used in the last vCPU time slice, set the lbr
>> stack to be interceptible, so that the host can capture whether the
>> lbr feature will be used in this time slice;
>> - if the lbr feature wasn't used in the last vCPU time slice, disable
>> the vCPU support of the guest lbr switching.
>>
>> Upon the first access to one of the lbr related MSRs (since the vCPU was
>> scheduled in):
>> - record that the guest has used the lbr;
>> - create a host perf event to help save/restore the guest lbr stack;
>> - pass the stack through to the guest.
> I don't understand a word of that.
>
> Who cares if the LBR MSRs are touched; the guest expects up-to-date
> values when it does reads them.
Another host thread who shares the same pCPU with this vCPU thread
may use the lbr stack, so the host needs to save/restore the vCPU's lbr
state.
Otherwise the guest perf inside the vCPU wouldn't read the correct data
from the lbr msr (as the msrs are changed by another host thread already).
As Andi also replied, if the vCPU isn't using lbr anymore, host doesn't need
to save the lbr msr then.
Best,
Wei
next prev parent reply other threads:[~2019-07-09 3:09 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-08 1:23 [PATCH v7 00/12] Guest LBR Enabling Wei Wang
2019-07-08 1:23 ` [PATCH v7 01/12] perf/x86: fix the variable type of the LBR MSRs Wei Wang
2019-07-08 1:23 ` [PATCH v7 02/12] perf/x86: add a function to get the lbr stack Wei Wang
2019-07-08 1:23 ` [PATCH v7 03/12] KVM/x86: KVM_CAP_X86_GUEST_LBR Wei Wang
2019-07-08 1:23 ` [PATCH v7 04/12] KVM/x86: intel_pmu_lbr_enable Wei Wang
2019-07-08 1:23 ` [PATCH v7 05/12] KVM/x86/vPMU: tweak kvm_pmu_get_msr Wei Wang
2019-07-08 1:23 ` [PATCH v7 06/12] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to the guest Wei Wang
2019-07-08 1:23 ` [PATCH v7 07/12] perf/x86: no counter allocation support Wei Wang
2019-07-08 14:29 ` Peter Zijlstra
2019-07-09 2:58 ` Wei Wang
2019-07-09 9:43 ` Peter Zijlstra
2019-07-09 11:36 ` Wei Wang
2019-07-08 1:23 ` [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host save/restore the guest lbr stack Wei Wang
2019-07-08 14:48 ` Peter Zijlstra
2019-07-09 3:04 ` Wei Wang
2019-07-09 9:39 ` Peter Zijlstra
2019-07-09 11:34 ` Wei Wang
2019-07-09 12:19 ` Peter Zijlstra
2019-07-10 8:19 ` Wei Wang
2019-07-09 11:45 ` Peter Zijlstra
2019-07-10 8:21 ` Wei Wang
2019-07-08 1:23 ` [PATCH v7 09/12] perf/x86: save/restore LBR_SELECT on vCPU switching Wei Wang
2019-07-08 1:23 ` [PATCH v7 10/12] KVM/x86/lbr: lazy save the guest lbr stack Wei Wang
2019-07-08 14:53 ` Peter Zijlstra
2019-07-08 15:11 ` Andi Kleen
2019-07-09 11:39 ` Peter Zijlstra
2019-07-09 3:14 ` Wei Wang [this message]
2019-07-08 1:23 ` [PATCH v7 11/12] KVM/x86: remove the common handling of the debugctl msr Wei Wang
2019-07-08 1:23 ` [PATCH v7 12/12] KVM/VMX/vPMU: support to report GLOBAL_STATUS_LBRS_FROZEN Wei Wang
2019-07-08 15:09 ` Peter Zijlstra
2019-07-09 3:24 ` Wei Wang
2019-07-09 11:35 ` Peter Zijlstra
2019-07-10 9:23 ` Wei Wang
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