linux-kernel - Re: [RESEND PATCH 01/13] KVM: x86: Relocate MMIO exit stats counting

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Date:   Fri, 23 Aug 2019 07:37:55 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [RESEND PATCH 01/13] KVM: x86: Relocate MMIO exit stats counting

On Fri, Aug 23, 2019 at 11:15:18AM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@...el.com> writes:
> 
> > Move the stat.mmio_exits update into x86_emulate_instruction().  This is
> > both a bug fix, e.g. the current update flows will incorrectly increment
> > mmio_exits on emulation failure, and a preparatory change to set the
> > stage for eliminating EMULATE_DONE and company.
> >
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> 
> This, however, makes me wonder why this is handled in x86-specific code
> in the first place, can we just count KVM_EXIT_MMIO exits when handling
> KVM_RUN?

struct kvm_vcpu_stat is arch specific.  At a glance, everyone is counting
similar things, but often in slightly different ways.  E.g. PowerPC and
ARM count MMIO exits, but PowerPC counts all exits, ARM has separate
counters for in-kernel vs. userspace, and x86 counts only userspace.