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

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Date:	Thu, 17 Oct 2013 10:51:52 +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 11:41, Gleb Natapov <gleb@...hat.com> wrote:
> On Thu, Oct 17, 2013 at 10:33:33AM +0100, Jan Beulich wrote:
>> >>> 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.
>> 
> KVM obviously knows the complete state of virtual CPU. It can figure the
> situation above by looking at CS descriptor, not need to check
> is_long_mode() at all. Here is how emulator does it:

And again - no: The last floating point operation may have
happened in 32-bit user mode context, while the state saving
may happen when the guest is already back in 64-bit kernel
mode.

Jan

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