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 09:43:57 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Liran Alon <liran.alon@...cle.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 Thu, Dec 12, 2019 at 04:57:10PM +0100, Paolo Bonzini wrote:
> On 12/12/19 16:52, Liran Alon wrote:
> >>> virt_apic_accesses	-> vapic
> >> apicv
> > Frankly, I dislike APICv terminology. I prefer to enumerate the
> > various VMX features which are collectively called APICv by KVM. 
> > APICv currently represents in KVM terminology the combination of
> > APIC-register virtualization, virtual-interrupt-delivery and
> > posted-interrupts (See cpu_has_vmx_apicv()).
> > 
> > In fact, the coupling of “enable_apicv” module parameter have made me
> > multiple times to need to disable entire APICv features when there
> > for example was only a bug in posted-interrupts.
> > 
> > Even you got confused as virtualize-apic-access is not part of KVM’s
> > APICv terminology but rather it’s enablement depend on
> > flexpriority_enabled (See cpu_need_virtualize_apic_accesses()). i.e.
> > It can be used for faster intercept handling of accesses to guest
> > xAPIC MMIO page.
> 
> Right, I got confused with APIC-register virtualization.  Virtualize
> APIC accesses is another one I wouldn't bother putting in /proc/cpuinfo,
> since it's usually present together with flexpriority.

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.