linux-kernel - Re: [PATCH] KVM: X86: correct trace_kvm_pv_tlb

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Date:   Thu, 05 May 2022 10:09:56 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Jon Kohler <jon@...anix.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH] KVM: X86: correct trace_kvm_pv_tlb_flush stats

Jon Kohler <jon@...anix.com> writes:

>> On May 4, 2022, at 5:47 PM, Sean Christopherson <seanjc@...gle.com> wrote:
>> 

...

>
> The net problem here is really that the stat is likely incorrect; however,
> one other oddity I didn’t quite understand after looking into this is that
> the call site for all of this is in record_steal_time(), which is only called
> from vcpu_enter_guest(), and that is called *after*
> kvm_service_local_tlb_flush_requests(), which also calls
> kvm_vcpu_flush_tlb_guest() if request == KVM_REQ_TLB_FLUSH_GUEST
>
> That request may be there set from a few different places. 
>
> I don’t have any proof of this, but it seems to me like we might have a
> situation where we double flush?
>
> Put another way, I wonder if there is any sense behind maybe hoisting
> if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu)) up before
> Other tlb flushes, and have it clear the FLUSH_GUEST if it was set?

Indeed, if we move KVM_REQ_STEAL_UPDATE check/record_steal_time() call
in vcpu_enter_guest() before kvm_service_local_tlb_flush_requests(), we
can probably get aways with kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST,
vcpu) in record_steal_time() which would help to avoid double flushing.

-- 
Vitaly