linux-kernel - Re: [PATCH 7/9] KVM/VMX: Emulate MSR_IA32_ARCH

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Date:   Mon, 19 Feb 2018 13:35:29 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH 7/9] KVM/VMX: Emulate MSR_IA32_ARCH_CAPABILITIES

On Mon, 2018-02-19 at 14:10 +0100, Paolo Bonzini wrote:
> > Hardware seems like a reasonable place to get the default value (cf.
> > the VMX capability MSRs).
> 
> There are some differences:
> 
> - a zero value for ARCH_CAPABILITIES should be safe, while a zero value
> for VMX capabilities doesn't really make sense.  On the contrary, a
> nonzero value for ARCH_CAPABILITIES is not safe across live migration.

Any VMM which is going to support live migration surely needs to pay at
least a small amount of attention to the features it exposes? Exposing
the ARCH_CAPABILITIES CPUID bit without actually looking at the
contents of the associated MSR which that bit advertises would be... a
little strange, would it not? 

I don't see why we care so much about the *default* value, in that
context.
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