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+CyuQCrfOQAv0PNgyuL68u2Xn7fXRqZCYE79wBZVDxVXng@mail.gmail.com>
Date:   Wed, 12 Jun 2019 10:20:14 +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.

Your design heavily depends on read host MSR_CORE_C1_RES for C1
residency msr emulation, however, the host MSR_CORE_C1_RES is
unreadable.
#rdmsr 0x660
rdmsr: CPU 0 cannot read MSR 0x00000660

Refer to turbostat codes,
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/power/x86/turbostat/turbostat.c#n1335
C1 is derivated from other parameters. use_c1_residency_msr is true
just for ATOM platform.

Regards,
Wanpeng Li