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Date: Thu, 17 Oct 2013 10:33:33 +0100
From: "Jan Beulich" <JBeulich@suse.com>
To: "Gleb Natapov" <gleb@redhat.com>
Cc: "Ingo Molnar" <mingo@elte.hu>, "Thomas Gleixner" <tglx@linutronix.de>,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        "KVM list" <kvm@vger.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        "Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH, RFC] x86-64: properly handle FPU code/data
 selectors
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>>> On 17.10.13 at 11:27, Gleb Natapov <gleb@redhat.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