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

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Date:   Mon, 19 Feb 2018 15:07:11 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        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 19/02/2018 14:35, David Woodhouse wrote:
> 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 think what we should do is simply backport Tom Lendacky's series to
4.14 and 4.9 ASAP, and add ARCH_CAPABILITIES support there.  Then the
question of the default becomes moot, more or less.

Paolo

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