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Message-ID: <5D25A019.6080708@intel.com>
Date: Wed, 10 Jul 2019 16:21:45 +0800
From: Wei Wang <wei.w.wang@...el.com>
To: Peter Zijlstra <peterz@...radead.org>
CC: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
pbonzini@...hat.com, ak@...ux.intel.com, kan.liang@...el.com,
mingo@...hat.com, rkrcmar@...hat.com, like.xu@...el.com,
jannh@...gle.com, arei.gonglei@...wei.com, jmattson@...gle.com
Subject: Re: [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host save/restore
the guest lbr stack
On 07/09/2019 07:45 PM, Peter Zijlstra wrote:
>
>> + * config: Actually this field won't be used by the perf core
>> + * as this event doesn't have a perf counter.
>> + * sample_period: Same as above.
> If it's unused; why do we need to set it at all?
OK, we'll remove the unused fields.
>
>> + * sample_type: tells the perf core that it is an lbr event.
>> + * branch_sample_type: tells the perf core that the lbr event works in
>> + * the user callstack mode so that the lbr stack will be
>> + * saved/restored on vCPU switching.
> Again; doesn't make sense. What does the user part have to do with
> save/restore? What happens when this vcpu thread drops to userspace for
> an assist?
This is a fake event which doesn't run the lbr on the host.
Returning to userspace doesn't need save/restore lbr, because userspace
wouldn't change those lbr msrs.
The event is created to save/restore lbr msrs on vcpu switching.
Host perf only do this save/restore for "user callstack mode" lbr event,
so we
construct the event to be "user callstack mode" to reuse the host lbr
save/restore.
An alternative method is to have KVM vcpu sched_in/out callback
save/restore those msrs, which don't need to create this fake event.
Please let me know if you prefer the second method.
Best,
Wei
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