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From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Borislav Petkov <bp@alien8.de>
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 09/19] x86/cpu: Clear VMX feature flag if VMX is not fully enabled
Date: Thu, 21 Nov 2019 13:07:14 -0800
Message-ID: <20191121210714.GB16617@linux.intel.com> (raw)
In-Reply-To: <20191121162452.GJ6540@zn.tnic>

On Thu, Nov 21, 2019 at 05:24:52PM +0100, Borislav Petkov wrote:
> On Mon, Nov 18, 2019 at 07:12:30PM -0800, Sean Christopherson wrote:
> > Now that the IA32_FEATURE_CONTROL MSR is guaranteed to be configured and
> > locked, clear the VMX capability flag if the IA32_FEATURE_CONTROL MSR is
> > not supported or if BIOS disabled VMX, i.e. locked IA32_FEATURE_CONTROL
> > and did not set the appropriate VMX enable bit.
> > 
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > ---
> >  arch/x86/kernel/cpu/feature_control.c | 28 ++++++++++++++++++++++++---
> >  1 file changed, 25 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/feature_control.c b/arch/x86/kernel/cpu/feature_control.c
> > index 33c9444dda52..2bd1a9e6021a 100644
> > --- a/arch/x86/kernel/cpu/feature_control.c
> > +++ b/arch/x86/kernel/cpu/feature_control.c
> > @@ -5,15 +5,26 @@
> >  #include <asm/msr-index.h>
> >  #include <asm/processor.h>
> >  
> > +#undef pr_fmt
> > +#define pr_fmt(fmt)	"x86/cpu: " fmt
> > +
> > +#define FEAT_CTL_UNSUPPORTED_MSG "IA32_FEATURE_CONTROL MSR unsupported on VMX capable CPU, suspected hardware or hypervisor issue.\n"
> > +
> >  void init_feature_control_msr(struct cpuinfo_x86 *c)
> >  {
> > +	bool tboot = tboot_enabled();
> >  	u64 msr;
> >  
> > -	if (rdmsrl_safe(MSR_IA32_FEATURE_CONTROL, &msr))
> > +	if (rdmsrl_safe(MSR_IA32_FEATURE_CONTROL, &msr)) {
> > +		if (cpu_has(c, X86_FEATURE_VMX)) {
> > +			pr_err_once(FEAT_CTL_UNSUPPORTED_MSG);
> > +			clear_cpu_cap(c, X86_FEATURE_VMX);
> > +		}
> >  		return;
> > +	}
> 
> Right, so this test: is this something that could happen on some
> configurations - i.e., the MSR is not there but VMX bit is set - or are
> you being too cautious here?

Probably being overly cautious.

> IOW, do you have any concrete use cases in mind (cloud provider can f*ck
> it up this way) or?

Yes, VMM somehow managing to break things.  Admittedly extremely unlikely
given how long IA32_FEATURE_CONTROL has been around.

> My angle is that if this is never going to happen, why even bother to
> print anything...

My thought was to add an equivalent of the WARN that fires when an MSR
access unexpectedly faults.  That's effectively what'd be happening, except
I used the safe variant to reduce the maintenance cost, e.g. so that the
RDMSR doesn't have to be conditioned on every possible feature.

What about a WARN_ON cpu_has?  That'd be more aligned with the unexpected
#GP on RDMSR behavior.

	if (rdmsrl_safe(...)) {
		if (WARN_ON_ONCE(cpu_has(c, X86_FEATURE_VMX)))
			clear_cpu_cap(c, X86_FEATURE_VMX);
		return;
	}

I'm also ok dropping it altogether, though from a KVM developer
perspective I wouldn't mind the extra sanity check :-)

  reply index

Thread overview: 42+ 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
2019-11-21 16:24   ` Borislav Petkov
2019-11-21 21:07     ` Sean Christopherson [this message]
2019-11-22 16:02       ` Borislav Petkov
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
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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