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

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Date:   Wed, 12 Jun 2019 10:04:04 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        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 Wed, Jun 12, 2019 at 07:10:49AM -0700, Sean Christopherson wrote:
> On Wed, Jun 12, 2019 at 06:02:59AM +0200, Borislav Petkov wrote:
> > On Tue, Jun 11, 2019 at 08:32:59PM -0700, Fenghua Yu wrote:
> > > Currently KVM doesn't simulate scattered features (the ones in CPUID_LNX_*
> > > in cpuid_leafs) as reverse_cpuid[] doesn't contain CPUID_LNX_*.
> > 
> > 43500e6f294d ("x86/cpufeatures: Remove get_scattered_cpuid_leaf()")
> > 
> > > 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.
> > 
> > Does KVM even support resctrl? I doubt only exporting a couple of CPUID
> > bits into the guest is enough...
> 
> KVM doesn't support resctrl.  Moving X86_FEATURES_CQM_* to a scattered
> leaf won't affect KVM.
> 
> > > Should patch #1 simulate X86_FEATURE_CQM_* in KVM? Or let KVM guys handle
> > > the scattered features?
> > 
> > Right, the scattered thing was removed as KVM didn't need it,
> > apparently, see above.
> 
> Let KVM people deal with it, in the unlikely event someone adds resctrl
> support to KVM.

Thank you for your insight, Boris and Sean!

-Fenghua