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

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Date:	Thu, 17 Oct 2013 11:37:48 +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 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.

Jan

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