Linux patch disabling AVX when eagerfpu is turned off - possibly not required

Linux patch disabling AVX when eagerfpu is turned off - possibly not required

[Leonid Shatz]

Regarding the patch disabling AVX when eagerfpu is off:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=3
94db20ca240741a08d472173db13d6f6a6e5a28

We have a suggestion that Intel documentation related to usage of CR0.TS bit
may be not properly updated with additional support of AVX/AVX2/etc
instructions. In fact, there is certain ambiguity about the causes of #NM
exception in other sections of Intel documentation. Based on the chapter 2.4
INSTRUCTION EXCEPTION SPECIFICATION of Intel 64 and IA-32 Architectures
Software Developer's Manual, all SSE/SSE2/SSE4/MMX/AVX/etc. instructions,
which modify the FPU/MMX/AVX state are supposed to generate #NM exception
when CR0.TS = 1.

In addition, based on AMD documentation, the #NM exception is generated for
both SSE and AVX instructions in protected mode - refer to AMD64
Architecture Programmer's Manual Volume 4: 128-Bit and 256-Bit Media
Instructions and also "AMD64 Architecture
Programmer's Manual Volume 2: System Programming" Chapter 3.1 (Task Switched
(TS) Bit.3. When an attempt is made to execute an x87 or media instruction
while TS=1, a device-not-available exception (#NM) occurs).

Also, in our limited internal test on Intel I7 processor (repeated on two
different machines), while executing the AVX VMOVAPS instruction with CR0.TS
bit set, we've observed the #NM exception being always generated (although
we didn't perform all-inclusive tests for entire set of AVX and similar new
instructions).

In view of above findings we would like to suggest to double check if
disabling AVX together with "eagerfpu off" is actually required and is a
real necessity. It would be helpful to consult with Intel engineers
regarding related design details.

Sincerely,
Leonid Shatz,
Seniour Software Enigneer, Hypervisor Team,
Ravello Systems, Inc.

Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required

[Dave Hansen]

On 01/21/2016 02:33 AM, Leonid Shatz wrote:
> In view of above findings we would like to suggest to double check if
> disabling AVX together with "eagerfpu off" is actually required and is a
> real necessity. It would be helpful to consult with Intel engineers
> regarding related design details.

Hi Leonid,

Thanks for the report!

Are you aware of any actual eagerfpu=off use in practice, or is this
mostly a theoretical concern?

Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required

[H. Peter Anvin]

On 01/21/16 12:08, Dave Hansen wrote:
> On 01/21/2016 02:33 AM, Leonid Shatz wrote:
>> In view of above findings we would like to suggest to double check if
>> disabling AVX together with "eagerfpu off" is actually required and is a
>> real necessity. It would be helpful to consult with Intel engineers
>> regarding related design details.
> 
> Hi Leonid,
> 
> Thanks for the report!
> 
> Are you aware of any actual eagerfpu=off use in practice, or is this
> mostly a theoretical concern?
> 

Looking into this, FWIW.

	-hpa

Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required

[Andy Lutomirski]

On Jan 21, 2016 12:12 PM, "H. Peter Anvin" <hpa@zytor.com> wrote:
>
> On 01/21/16 12:08, Dave Hansen wrote:
> > On 01/21/2016 02:33 AM, Leonid Shatz wrote:
> >> In view of above findings we would like to suggest to double check if
> >> disabling AVX together with "eagerfpu off" is actually required and is a
> >> real necessity. It would be helpful to consult with Intel engineers
> >> regarding related design details.
> >
> > Hi Leonid,
> >
> > Thanks for the report!
> >
> > Are you aware of any actual eagerfpu=off use in practice, or is this
> > mostly a theoretical concern?
> >
>
> Looking into this, FWIW.

I still think that we should default eagerfpu=on on all CPUs for one
release and then, when nothing breaks, delete all the old lazy code.
I'll respin and send that patch.

--Andy

Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required

[Yu-cheng Yu]

Hello Leonid,

You are probably right. Let me check this and get back to you.

Thanks,
Yu-cheng

Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required

[H. Peter Anvin]

On 01/21/16 13:03, Yu-cheng Yu wrote:
> Hello Leonid,
> 
> You are probably right. Let me check this and get back to you.
> 
> Thanks,
> Yu-cheng
> 

The hardware people have gotten back to us and this inconsistency in the
SDM is unintentional (other places in the SDM has it correctly.)  So we
should revert this patch.

	-hpa

Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required

[Yu-cheng Yu]

On Thu, Jan 21, 2016 at 01:08:18PM -0800, H. Peter Anvin wrote:
> On 01/21/16 13:03, Yu-cheng Yu wrote:
> > Hello Leonid,
> > 
> > You are probably right. Let me check this and get back to you.
> > 
> > Thanks,
> > Yu-cheng
> > 
> 
> The hardware people have gotten back to us and this inconsistency in the
> SDM is unintentional (other places in the SDM has it correctly.)  So we
> should revert this patch.
> 
> 	-hpa
> 
Agree. Thanks Peter and Leonid!

RE: Linux patch disabling AVX when eagerfpu is turned off - possibly not required

[Leonid Shatz]

On Thu, Jan 21, 2016 at 01:08:18PM -0800, H. Peter Anvin wrote:
>> On 01/21/16 13:03, Yu-cheng Yu wrote:
>> > Hello Leonid,
>> > 
>> > You are probably right. Let me check this and get back to you.
>> > 
>> > Thanks,
>> > Yu-cheng
>> > 
>> 
>> The hardware people have gotten back to us and this inconsistency in 
>> the SDM is unintentional (other places in the SDM has it correctly.)  
>> So we should revert this patch.
>> 
>> 	-hpa
>> 
>Agree. Thanks Peter and Leonid!

Thank you all for your feedback and clarifications. That's was my assumption
that SDM was somewhat misleading, but I was not %100 sure about it. It could
be nice, though to see future releases of SDM taking this issue in account.

Leonid

Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required

[H. Peter Anvin]

On 01/21/16 13:19, Leonid Shatz wrote:
> 
> Thank you all for your feedback and clarifications. That's was my assumption
> that SDM was somewhat misleading, but I was not %100 sure about it. It could
> be nice, though to see future releases of SDM taking this issue in account.
> 

It will.

	-hpa