linux-kernel - Re: [PATCH] KVM: x86: emulate SAHF instruction

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Message-ID: <52726DE0.3060207@redhat.com>
Date:	Thu, 31 Oct 2013 15:49:04 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: emulate SAHF instruction

Il 31/10/2013 15:34, Gleb Natapov ha scritto:
> I haven't checked AMD doc, but if it is documented that lahf/sahf #UDs at 64
> bit we should emulate it correctly.

It says "The LAHF instruction can only be executed in 64-bit mode if
supported by the processor implementation. Check the status of ECX bit 0
returned by CPUID function 8000_0001h to verify that the processor
supports LAHF in 64-bit mode".  Same as Intel---in fact 80000001h is an
"AMD leaf" so to speak.

I found "AMD introduced support for the instructions with their Athlon
64, Opteron and Turion 64 revision D processors in March 2005 and Intel
introduced support for the instructions with the Pentium 4 G1 stepping
in December 2005".  I think we can for all practical purposes ignore the
lahf_lm CPUID flag.

> Who knows what code depends on it.
> Of course I pretty much doubt we will ever emulate sahf in 64 bit mode

Yep.

Paolo

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