linux-kernel - Re: [PATCH v2 2/3] KVM: X86: Provide a capability to disable cstate msr read intercepts

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Message-ID: <CANRm+Cy+BVHeXjfSPhPz=n7_Qg8oQEC8DdcpCzEV5v4qYXgJGw@mail.gmail.com>
Date:   Tue, 11 Jun 2019 19:35:11 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 2/3] KVM: X86: Provide a capability to disable cstate
 msr read intercepts

On Tue, 11 Jun 2019 at 19:09, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 11/06/19 09:38, Wanpeng Li wrote:
> > MSR_CORE_C1_RES is unreadable except for ATOM platform, so I think we
> > can avoid the complex logic to handle C1 now. :)
>
> I disagree.  Linux uses it on all platforms is available, and virtual
> machines that don't pass mwait through _only_ have C1, so it would be
> less useful to have deep C-state residency MSRs and not C1 residency.
>
> But turbostat can get the information from sysfs, so what are these MSRs
> used for?

The sysfs is not accurate, the time which is accumulated in the
function cpuidle_enter_state() is the expected cstate we hope to enter
instead of the real cstate we finally enter. For example, we found
several SKX/CLX models can't enter deeper cstates in non-root mode,
Intel hardware team has been working on this according to our report
recently. The bare-metal cstate residency msrs don't increase in
non-root mode, however, the time under
/sys/devices/system/cpu/cpux/cpuidle/statex/ increase gradually in the
guest.

Regards,
Wanpeng Li