linux-kernel - Re: [PATCH v4 11/19] x86/cpu: Print VMX flags in /proc/cpuinfo using VMX_FEATURES

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Date:   Thu, 12 Dec 2019 10:27:21 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Tony Luck <tony.luck@...el.com>,
        Tony W Wang-oc <TonyWWang-oc@...oxin.com>,
        Len Brown <lenb@...nel.org>, Shuah Khan <shuah@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>, linux-edac@...r.kernel.org,
        Linux PM list <linux-pm@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Subject: Re: [PATCH v4 11/19] x86/cpu: Print VMX flags in /proc/cpuinfo using
 VMX_FEATURES_*

On Thu, Dec 12, 2019 at 08:04:19PM +0200, Liran Alon wrote:
> 
> 
> > On 12 Dec 2019, at 19:57, Jim Mattson <jmattson@...gle.com> wrote:
> > 
> > On Thu, Dec 12, 2019 at 9:53 AM Liran Alon <liran.alon@...cle.com> wrote:
> > 
> >> Why should CPU VMX features be treated differently than standard CPUID deduced features?
> > 
> > Do we have the right Intel people on the recipient list to answer this
> > question? Presumably, Intel felt that this information should be
> > available in supervisor mode only.
> > 
> > Sean?
> 
> Good question. Probably because it just makes sense that Ring3 will never need to use
> this info as all VMX instructions are privileged. i.e. Can only be executed in Ring0.

I highly doubt ring0 vs. ring3 was a motivating factor.  I suspect the MSR
interface is primarily driven by VMX's allowed-0 vs. allowed-1 behavior,
which would be awkward to encode in CPUID.  Reporting via MSR also likely
provided more flexibility for updating/fixing CPU behavior, e.g. patching
the RDMSR hook is likely far easier than patching CPUID.

Even if the architects intended the information to be supervisor-only,
that's just their opinion, no?

> De-facto in KVM we have discovered this assumption to be problematic BTW,
> as KVM created an interface to query VMX MSRs values to properly define the requested
> vCPU model. :P (See kvm_get_msr_feature())