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

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Message-ID: <Y1FatU6Yf9n5pWB+@google.com>
Date:   Thu, 20 Oct 2022 14:27:01 +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 Thu, Oct 20, 2022, Xiaoyao Li wrote:
> vcpu->arch.mcg_cap represents the value of MSR_IA32_MCG_CAP. It's
> set via ioctl(KVM_X86_SETUP_MCE) from userspace when exposing and
> configuring MCE to guest.
> 
> It's wrong to leave the default value as KVM_MAX_MCE_BANKS.

Why?  I agree it's an odd default, but the whole MCE API is odd.  Functionally,
I don't see anything that's broken by allowing the guest to access the MCx_CTL MSRs
by default.