linux-kernel - Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors

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Message-ID: <20131017092727.GN15657@redhat.com>
Date:	Thu, 17 Oct 2013 12:27:27 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	KVM list <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors

On Wed, Oct 16, 2013 at 05:13:40PM +0100, Jan Beulich wrote:
> > It preserves *less* state, because the upper 32 bits of rip are now
> > corrupted. Any 64-bit application that actually looks at the FP
> > rip/rdp fields now get the WRONG VALUES.
> 
> But again - this isn't being done for ordinary 64-bit applications,
> this is only happening for KVM guests. And there not being a
> protocol for telling the caller whether a certain context hold
> 64-bit offsets or selector/offset pairs shouldn't be a reason to
> think of a solution to the problem.
> 
KVM knows what mode guest vcpu is in. is_long_mode(vcpu) will tell you
if it is in long mode or not. No need to guess it.

--
			Gleb.
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