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From: Borislav Petkov <bp@alien8.de>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Arnaldo Carvalho de Melo" <acme@kernel.org>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"Alexander Shishkin" <alexander.shishkin@linux.intel.com>,
	"Jiri Olsa" <jolsa@redhat.com>,
	"Namhyung Kim" <namhyung@kernel.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Jim Mattson" <jmattson@google.com>,
	"Joerg Roedel" <joro@8bytes.org>,
	"Tony Luck" <tony.luck@intel.com>,
	"Tony W Wang-oc" <TonyWWang-oc@zhaoxin.com>,
	"Shuah Khan" <shuah@kernel.org>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	linux-edac@vger.kernel.org, linux-kselftest@vger.kernel.org,
	"Jarkko Sakkinen" <jarkko.sakkinen@linux.intel.com>
Subject: Re: [PATCH v3 12/19] x86/vmx: Introduce VMX_FEATURES_*
Date: Fri, 22 Nov 2019 19:36:41 +0100
Message-ID: <20191122183641.GJ6289@zn.tnic> (raw)
In-Reply-To: <20191121215017.GC16617@linux.intel.com>

On Thu, Nov 21, 2019 at 01:50:17PM -0800, Sean Christopherson wrote:
> I actually don't want to use cpu_has() for the VMX features, which is
> why I put these in a separate array (one of the future patches).
>  
> The motivation is purely for /proc/cpuinfo.  Currently there is no sane
> way for a user to query the capabilities of their platform.  The issue
> comes up on a fairly regular basis, e.g. when trying to find a platform
> to test a new feature or to debug an issue that has a hardware dependency.
> 
> Lack of reporting is especially annoying when the end user isn't familiar
> with VMX, e.g. the format of the MSRs is non-standard, existence of some
> MSRs is reported by bits in other MSRs, several features from KVM's point
> of view are actually enumerated as 3+ separate features by hardware, etc...
> 
> Punting to a userspace utility isn't a viable option because all of the
> capabilities are reported via MSRs.

Ok, this justification was missing in the commit message, please add the
gist of it so that it is clear why we're doing it. And yes, I agree that
having a single concentrated place for feature bits which you otherwise
have to painstakingly extract from a bunch of MSRs make sense.

> As for why I want to keep these out of cpu_has()... VMX has a concept of
> features being fixed "on", e.g. early CPUs don't allow disabling off CR3
> interception.  A cpu_has() approach doesn't work well since it loses the
> information regarding which bits are fixed-1.  KVM also has several module
> params that can be used to disable use of features, i.e. we don't want
> cpu_has() for VMX features because the KVM-specific variables need to be
> the canonical reference.

Well, you can use the cpu_has() machinery for stuff like that too - we
can clear bits there too: clear_cpu_cap() - and since clearing those
bits are only for /proc/cpuinfo reporting, it's not like anything would
break if that flag is gone. Just saying, in case you want to use the
machinery for that.

And that would avoid some of the duplication of having KVM-specific
variables *and* VMX_FEATURE_* flags, where latter are not really
toggleable but only for /proc/cpuinfo. Especially if you wanna enforce
"developers to define a VMX_FEATURE flag when adding support for a new
hardware feature."

> Alternatively, what about adding "vmx flags" but keeping the existing
> synthetic flags?  That'd mean having duplicates for tpr_shadow, vnmi, ept,
> flexpriority, vpi and ept_ad.  On the plus side, we'd cap the pollution of
> "flags" at those six features.

Yap, this is how you avoid ABI breakage. Makes sense to me.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

  reply index

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-19  3:12 [PATCH v3 00/19] x86/cpu: Clean up handling of VMX features Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 01/19] x86/msr-index: Clean up bit defines for IA32_FEATURE_CONTROL MSR Sean Christopherson
2019-11-19 11:15   ` Borislav Petkov
2019-11-19 23:18     ` Sean Christopherson
2019-11-20 17:48       ` Borislav Petkov
2019-11-21  9:46   ` Jarkko Sakkinen
2019-11-21 22:14     ` Sean Christopherson
2019-11-29 21:06       ` Jarkko Sakkinen
2019-11-29 21:11         ` Jarkko Sakkinen
2019-11-19  3:12 ` [PATCH v3 02/19] selftests: kvm: Replace manual MSR defs with common msr-index.h Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 03/19] tools arch x86: Sync msr-index.h from kernel sources Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 04/19] x86/intel: Initialize IA32_FEATURE_CONTROL MSR at boot Sean Christopherson
2019-11-19  4:41   ` Kai Huang
2019-11-19  5:03     ` Sean Christopherson
2019-11-21 10:39   ` Jarkko Sakkinen
2019-11-21 10:41     ` Jarkko Sakkinen
2019-11-21 11:05       ` Borislav Petkov
2019-11-21 22:12         ` Sean Christopherson
2019-11-22 12:34           ` Borislav Petkov
2019-11-19  3:12 ` [PATCH v3 05/19] x86/mce: WARN once if IA32_FEATURE_CONTROL MSR is left unlocked Sean Christopherson
2019-11-21 10:45   ` Jarkko Sakkinen
2019-11-19  3:12 ` [PATCH v3 06/19] x86/centaur: Use common IA32_FEATURE_CONTROL MSR initialization Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 07/19] x86/zhaoxin: " Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 08/19] KVM: VMX: Drop initialization of IA32_FEATURE_CONTROL MSR Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 09/19] x86/cpu: Clear VMX feature flag if VMX is not fully enabled Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 10/19] KVM: VMX: Use VMX feature flag to query BIOS enabling Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 11/19] KVM: VMX: Check for full VMX support when verifying CPU compatibility Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 12/19] x86/vmx: Introduce VMX_FEATURES_* Sean Christopherson
2019-11-21 16:52   ` Borislav Petkov
2019-11-21 21:50     ` Sean Christopherson
2019-11-22 18:36       ` Borislav Petkov [this message]
2019-11-22 19:09         ` Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 13/19] x86/cpu: Detect VMX features on Intel, Centaur and Zhaoxin CPUs Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 14/19] x86/cpu: Print VMX flags in /proc/cpuinfo using VMX_FEATURES_* Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 15/19] x86/cpufeatures: Drop synthetic VMX feature flags Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 16/19] KVM: VMX: Use VMX_FEATURE_* flags to define VMCS control bits Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 17/19] x86/cpufeatures: Clean up synthetic virtualization flags Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 18/19] perf/x86: Provide stubs of KVM helpers for non-Intel CPUs Sean Christopherson
2019-11-19  3:12 ` [PATCH v3 19/19] KVM: VMX: Allow KVM_INTEL when building for Centaur and/or Zhaoxin CPUs Sean Christopherson

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