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 19:52:55 +0200
From:   Liran Alon <liran.alon@...cle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, 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>,
        Jim Mattson <jmattson@...gle.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>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-edac@...r.kernel.org, linux-pm@...r.kernel.org,
        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 12 Dec 2019, at 19:47, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> On 12/12/19 18:43, Sean Christopherson wrote:
>> Key word being "usually".  My intent in printing out partially redundant
>> flags was to help users debug/understand why the combined feature isn't
>> supported.  E.g. userspace can already easily (relatively speaking) query
>> flexpriority support via /sys/module/kvm_intel/parameters/flexpriority.
>> But if that comes back "N", the user has no way to determine exactly why
>> flexpriority is disabled.
> 
> There are tools such as vmxcap.  It is part of QEMU, but I wouldn't mind
> moving it into the kernel tree.
> 
> Paolo
> 

True. But it relies on /dev/msr0 (exposed from msr kernel module) to be able to read any host arbitrary MSR.
It’s much more elegant to just report CPU VMX features in a /proc/cpuinfo pseudo-file and don’t require strong privileges.
Why should CPU VMX features be treated differently than standard CPUID deduced features?
CPU SVM features are also present in /proc/cpuinfo today (Because they are deduced from CPUID leafs). So it should be similar.

-Liran