linux-kernel - Re: [PATCH] kvm: fix cpuid eax

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Message-ID: <20120430204126.GA14876@redhat.com>
Date:	Mon, 30 Apr 2012 23:41:27 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, davej@...hat.com
Subject: Re: [PATCH] kvm: fix cpuid eax

On Mon, Apr 30, 2012 at 03:37:14PM -0500, Anthony Liguori wrote:
> On 04/30/2012 09:39 AM, Michael S. Tsirkin wrote:
> >cpuid eax should return the max leaf so that
> >guests can find out the valid range.
> >This matches Xen et al.
> 
> What KVM does here predates Xen and Hyper-V.
> 
> This is an ABI breaker.

It is? I tested an old guest and qemu and both work fine.
What breaks?

> Regards,
> 
> Anthony Liguori
> 
> >
> >Tested using -cpu host.
> >
> >Signed-off-by: Michael S. Tsirkin<mst@...hat.com>
> >---
> >  arch/x86/kvm/cpuid.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> >
> >diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> >index 7d00d2d..bda4877 100644
> >--- a/arch/x86/kvm/cpuid.c
> >+++ b/arch/x86/kvm/cpuid.c
> >@@ -397,7 +397,7 @@ static int do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
> >  	case KVM_CPUID_SIGNATURE: {
> >  		char signature[12] = "KVMKVMKVM\0\0";
> >  		u32 *sigptr = (u32 *)signature;
> >-		entry->eax = 0;
> >+		entry->eax = KVM_CPUID_FEATURES;
> >  		entry->ebx = sigptr[0];
> >  		entry->ecx = sigptr[1];
> >  		entry->edx = sigptr[2];
--
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