linux-kernel - Re: [PATCH] KVM: x86: Fix the initial value of mcg

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Date:   Fri, 21 Oct 2022 18:35:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Fix the initial value of mcg_cap

On Fri, Oct 21, 2022, Xiaoyao Li wrote:
> On 10/21/2022 12:32 AM, Sean Christopherson wrote:
> > If we really want to clean up this code, I think the correct approach would be to
> > inject #GP on all relevant MSRs if CPUID.MCA==0, e.g.
> 
> It's what I thought of as well. But I didn't find any statement in SDM of
> "Accessing Machine Check MSRs gets #GP if no CPUID.MCA"

Ugh, stupid SDM.  Really old SDMs, e.g. circa 1997, explicity state in the
CPUID.MCA entry that:

  Processor supports the MCG_CAP MSR.

But, when Intel introduced the "Architectural MSRs" section (2001 or so), the
wording was changed to be less explicit:

  The Machine Check Architecture, which provides a compatible mechanism for error
  reporting in P6 family, Pentium 4, and Intel Xeon processors, and future processors,
  is supported. The MCG_CAP MSR contains feature bits describing how many banks of
  error reporting MSRs are supported.

and the entry in the MSR index just lists P6 as the dependency:

  IA32_MCG_CAP (MCG_CAP) Global Machine Check Capability (R/O) 06_01H

So I think it's technically true that MCG_CAP is supposed to exist iff CPUID.MCA=1,
but we'd probably need an SDM change to really be able to enforce that :-(