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Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM

From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled
Date: Mon, 27 May 2013 10:32:12 -0300
User-agent: Mutt/1.5.21 (2010-09-15)

On Mon, May 27, 2013 at 03:14:25PM +0200, Paolo Bonzini wrote:
> Il 27/05/2013 15:07, Eduardo Habkost ha scritto:
> >> Changing TCG to KVM should not change hardware
> >> > if you use "-cpu ...,enforce", so it is right that it fails when
> >> > starting with KVM.
> >> > 
> > Changing between KVM and TCG _does_ change hardware, today (with or
> > without check/enforce). All CPU models on TCG have features not
> > supported by TCG automatically removed. See the "if (!kvm_enabled())"
> > block at x86_cpu_realizefn().
> Perhaps (for "-cpu ...,enforce" or check) that's the real bug we have to
> fix?

It would be 100% accurate, but would it be useful? We will then have a
Opteron_G3 CPU model that can't be used reliably without low-level bit
fiddling, on either TCG or KVM mode.

Note that I am not completely against it. I agree that having a single
CPU model namespace with 100%-equivalent definitions for both TCG and
KVM modes would simplify the logic a lot. I am just not sure if it is
worth it.

(To be honest, I am more worried about the amount of time I will waste
trying to change the behavior of the TCG code. I am pretty sure I will
hear something like "why are you wasting your time trying to make
check/enforce work for TCG? Please leave TCG alone!".)


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