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

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Date:	Thu, 17 Oct 2013 13:39:56 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Jan Beulich <JBeulich@...e.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 Thu, Oct 17, 2013 at 11:37:48AM +0100, Jan Beulich wrote:
> >>> On 17.10.13 at 12:23, Gleb Natapov <gleb@...hat.com> wrote:
> > On Thu, Oct 17, 2013 at 10:51:52AM +0100, Jan Beulich wrote:
> >> >>> On 17.10.13 at 11:41, Gleb Natapov <gleb@...hat.com> wrote:
> >> > 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.
> >> 
> > Hmm, OK so the scenarios you are talking about is:
> >  1. Guest's 32bit process uses FPU
> >  2. Guest switch to 64bit kernel.
> >  3. Before guest's kernel saves 32bit process's FPU state VMEXIT happens
> >  4. KVM need to save FPU but it does not know what mode it is in
> > Correct? 
> 
> Yes.
> 
> > KVM gives FPU to a guest lazily, meaning that on a first FPU use #NM
> > (intercepted by KVM) happens at which point FPU is granted to a guest.
> > KVM can check what mode CPU was in at this point and use this info
> > while saving FPU. But there is additional optimization that will prevent
> > this from working for all cases: when FPU is granted to a guest KVM
> > disabled CR0.TS/#NM intercepts, so guest is free to switch FPU from
> > 32bit to 64bit mode without KVM knowing. Disabling this optimization
> > will make FP intensive workload slow in a guest.
> 
> Not sure what you're trying to tell me with this explanation.
> 
Trying to think aloud how it can be fixed.

--
			Gleb.
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