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

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Message-ID: <20190612141049.GA20308@linux.intel.com>
Date:   Wed, 12 Jun 2019 07:10:49 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        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 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.