linux-kernel - Re: [PATCH 4/5] export new cpuid KVM

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Message-ID: <4BCA04D2.1040300@redhat.com>
Date:	Sat, 17 Apr 2010 21:58:26 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] export new cpuid KVM_CAP

On 04/15/2010 09:37 PM, Glauber Costa wrote:
> Since we're changing the msrs kvmclock uses, we have to communicate
> that to the guest, through cpuid. We can add a new KVM_CAP to the
> hypervisor, and then patch userspace to recognize it.
>
> And if we ever add a new cpuid bit in the future, we have to do that again,
> which create some complexity and delay in feature adoption.
>
> Instead, what I'm proposing in this patch is a new capability, called
> KVM_CAP_X86_CPUID_FEATURE_LIST, that returns the current feature list
> currently supported by the hypervisor. If we ever want to add or remove
> some feature, we only need to tweak into the HV, leaving userspace untouched.
>
>    

Hm.  We need to update userspace anyway, since we don't like turning 
features on unconditionally (it breaks live migration into an older kernel).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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