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Groups > linux.kernel > #1314087 > unrolled thread
| Started by | "Leonid Shatz" <leonid.shatz@ravellosystems.com> |
|---|---|
| First post | 2016-01-21 11:40 +0100 |
| Last post | 2016-01-21 22:30 +0100 |
| Articles | 9 — 5 participants |
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Linux patch disabling AVX when eagerfpu is turned off - possibly not required "Leonid Shatz" <leonid.shatz@ravellosystems.com> - 2016-01-21 11:40 +0100
Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required Dave Hansen <dave.hansen@intel.com> - 2016-01-21 21:10 +0100
Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required "H. Peter Anvin" <hpa@zytor.com> - 2016-01-21 21:20 +0100
Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required Andy Lutomirski <luto@amacapital.net> - 2016-01-21 22:10 +0100
Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required Yu-cheng Yu <yu-cheng.yu@intel.com> - 2016-01-21 22:10 +0100
Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required "H. Peter Anvin" <hpa@zytor.com> - 2016-01-21 22:10 +0100
Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required Yu-cheng Yu <yu-cheng.yu@intel.com> - 2016-01-21 22:20 +0100
RE: Linux patch disabling AVX when eagerfpu is turned off - possibly not required "Leonid Shatz" <leonid.shatz@ravellosystems.com> - 2016-01-21 22:20 +0100
Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required "H. Peter Anvin" <hpa@zytor.com> - 2016-01-21 22:30 +0100
| From | "Leonid Shatz" <leonid.shatz@ravellosystems.com> |
|---|---|
| Date | 2016-01-21 11:40 +0100 |
| Subject | Linux patch disabling AVX when eagerfpu is turned off - possibly not required |
| Message-ID | <qTkOJ-3XA-11@gated-at.bofh.it> |
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.
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| From | Dave Hansen <dave.hansen@intel.com> |
|---|---|
| Date | 2016-01-21 21:10 +0100 |
| Subject | Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required |
| Message-ID | <qTtIm-1Np-17@gated-at.bofh.it> |
| In reply to | #1314087 |
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?
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| From | "H. Peter Anvin" <hpa@zytor.com> |
|---|---|
| Date | 2016-01-21 21:20 +0100 |
| Subject | Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required |
| Message-ID | <qTtS2-1R8-17@gated-at.bofh.it> |
| In reply to | #1314480 |
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
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| From | Andy Lutomirski <luto@amacapital.net> |
|---|---|
| Date | 2016-01-21 22:10 +0100 |
| Subject | Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required |
| Message-ID | <qTuEq-2ry-11@gated-at.bofh.it> |
| In reply to | #1314491 |
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
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| From | Yu-cheng Yu <yu-cheng.yu@intel.com> |
|---|---|
| Date | 2016-01-21 22:10 +0100 |
| Subject | Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required |
| Message-ID | <qTuEq-2ry-1@gated-at.bofh.it> |
| In reply to | #1314087 |
Hello Leonid, You are probably right. Let me check this and get back to you. Thanks, Yu-cheng
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| From | "H. Peter Anvin" <hpa@zytor.com> |
|---|---|
| Date | 2016-01-21 22:10 +0100 |
| Subject | Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required |
| Message-ID | <qTuEq-2ry-15@gated-at.bofh.it> |
| In reply to | #1314523 |
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
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| From | Yu-cheng Yu <yu-cheng.yu@intel.com> |
|---|---|
| Date | 2016-01-21 22:20 +0100 |
| Subject | Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required |
| Message-ID | <qTuO6-2vq-7@gated-at.bofh.it> |
| In reply to | #1314528 |
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!
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| From | "Leonid Shatz" <leonid.shatz@ravellosystems.com> |
|---|---|
| Date | 2016-01-21 22:20 +0100 |
| Message-ID | <qTuO6-2vq-5@gated-at.bofh.it> |
| In reply to | #1314533 |
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
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| From | "H. Peter Anvin" <hpa@zytor.com> |
|---|---|
| Date | 2016-01-21 22:30 +0100 |
| Subject | Re: Linux patch disabling AVX when eagerfpu is turned off - possibly not required |
| Message-ID | <qTuXM-2zU-7@gated-at.bofh.it> |
| In reply to | #1314534 |
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
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