linux-kernel - Re: [RFC PATCH] x86/cpufeatures: Enumerate new AVX512 bfloat16 instructions

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Date:   Tue, 11 Jun 2019 20:32:59 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH] x86/cpufeatures: Enumerate new AVX512 bfloat16
 instructions

On Tue, Jun 11, 2019 at 09:47:02PM +0200, Borislav Petkov wrote:
> On Tue, Jun 11, 2019 at 11:19:20AM -0700, Fenghua Yu wrote:
> > So can I re-organize word 11 and 12 as follows?
> > 
> > 1. Change word 11 to host scattered features.
> > 2. Move the previos features in word 11 and word 12 to word 11:
> > /*
> >  * Extended auxiliary flags: Linux defined - For features scattered in various
> >  * CPUID levels and sub-leaves like CPUID level 7 and sub-leaf 1, etc, word 19.
> >  */
> > #define X86_FEATURE_CQM_LLC             (11*32+ 0) /* LLC QoS if 1 */
> > #define X86_FEATURE_CQM_OCCUP_LLC       (11*32+ 1) /* LLC occupancy monitoring */
> > #define X86_FEATURE_CQM_MBM_TOTAL       (11*32+ 2) /* LLC Total MBM monitoring */
> > #define X86_FEATURE_CQM_MBM_LOCAL       (11*32+ 3) /* LLC Local MBM monitoring */
> 
> Yap.
> 
> > 3. Change word 12 to host CPUID.(EAX=7,ECX=1):EAX:
> > /* Intel-defined CPU features, CPUID level 0x7:1 (EAX), word 12 */
> > #define X86_FEATURE_AVX512_BF16         (12*32+ 0) /* BFLOAT16 instructions */
> 
> This needs to be (12*32+ 5) if word 12 is going to map leaf
> CPUID.(EAX=7,ECX=1):EAX.
> 
> At least judging from the arch extensions doc which lists EAX as:
> 
> Bits 04-00: Reserved.
> Bit 05: AVX512_BF16. Vector Neural Network Instructions supporting BFLOAT16 inputs and conversion instructions from IEEE single precision.
> Bits 31-06: Reserved.
> 
> > 4. Do other necessary changes to match the new word 11 and word 12.
> 
> But split in two patches: first does steps 1+2, second patch adds the
> new leaf to word 12.

Currently KVM doesn't simulate scattered features (the ones in CPUID_LNX_*
in cpuid_leafs) as reverse_cpuid[] doesn't contain CPUID_LNX_*.

After the X86_FEATURES_CQM_* features are changed to scattered features,
they will not be simulated by KVM any more as CPUID_F_0_EDX and CPUID_F_1_EDX
are removed.

Should patch #1 simulate X86_FEATURE_CQM_* in KVM? Or let KVM guys handle
the scattered features?

Thanks.

-Fenghua