Re: [PATCH v5 13/19] x86/cpufeatures: Add flag to track whether MSR IA32_FEAT_CTL is configured

Sean Christopherson
2020-02-26T00:43:04Z

On Tue, Feb 25, 2020 at 03:54:34PM -0800, Jacob Keller wrote:
> On 2/25/2020 3:29 PM, Sean Christopherson wrote:
> > On Tue, Feb 25, 2020 at 02:52:32PM -0800, Jacob Keller wrote:
> >> On 2/25/2020 2:12 PM, Sean Christopherson wrote:
> >>> On Tue, Feb 25, 2020 at 01:49:13PM -0800, Jacob Keller wrote:
> >>>> Hi Sean,
> >>>>
> >>>> I suspect something is wrong and the features are enabled even though
> >>>> the BIOS has it disabled, leading to later failure because of this.
> >>>
> >>> Hrm. On the failing kernel, what are the values of MSR 0x3a for all CPUs,
> >>> i.e. what's the output of 'sudo rdmsr -a 0x3a'?
> >>>
> >>
> >> On the old (fedora 30) kernel, every cpu reports as '1'.
> >>
> >> I can't easily test the failing kernel because it crashes during boot.
> >
> > No need, your BIOS is likely locking the MSR, I doubt the value is any
> > different when running the new kernel.
> >
> > Does reverting commit a4d0b2fdbcf7 ("KVM: VMX: Use VMX feature flag to
> > query BIOS enabling") resolve the issue?
> >
> > Is the failing kernel an (umodified) upstream kernel? A stable kernel?
> > Or something else? Assuming it's an unmodified upstream kernel, can you
> > send your .config? I've tried all the obvious Kconfig combinations but
> > haven't been able to reproduce the problem. Staring at the code hasn't
> > yielded any revelations either.
> >
>
> I reverted the suggested commit and added some prints:
>
> [ 26.056398] X86_FEATURE_MSR_IA32_FEAT_CTL is enabled
> [ 26.062426] X86_FEATURE_VMX is enabled
> [ 26.066923] kvm: disabled by bios
>
> So the old code flow is finding KVM to be disabled, but both features
> are set...
>
> The code that sets this is run first:
>
> > Feb 25 15:46:05 jbrandeb-saw1 kernel: x86/cpu: FEAT_CTL_LOCKED is set
> > Feb 25 15:46:05 jbrandeb-saw1 kernel: x86/cpu: FEAT_CTL_VMX_ENABLED_INSIDE_SMX is unset
> > Feb 25 15:46:05 jbrandeb-saw1 kernel: x86/cpu: FEAT_CTL_VMX_ENABLED_OUTSIDE_SMX is unset
> > Feb 25 15:46:05 jbrandeb-saw1 kernel: x86/cpu: MSR locked by bios
> > Feb 25 15:46:05 jbrandeb-saw1 kernel: x86/cpu: VMX (outside TXT) disabled by BIOS
> > Feb 25 15:46:05 jbrandeb-saw1 kernel: x86/cpu: disabling X86_FEATURE_VMX
>
> But somehow... it is still set later...
>
> So there's something weird going on. Maybe "boot_cpu_has" in the
> vmx_disabled_by_bios is wrong? Hmm.

Hmm, perhaps a bug somewhere else is overwriting the cpufeatures bit for
X86_FEATURE_VMX. Let me see if I can reproduce from net-next.