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

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Date:	Thu, 17 Oct 2013 10:33:33 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Gleb Natapov" <gleb@...hat.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"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 17.10.13 at 11:27, Gleb Natapov <gleb@...hat.com> wrote:
> 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.

So what if that 64-bit guest OS is running a 32-bit app? You can
only positively know the _current_ guest word size when the
guest is not in long mode.

Jan

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