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

Sean Christopherson
2020-02-26T20:57:47Z

On Wed, Feb 26, 2020 at 12:41:09PM -0800, Jacob Keller wrote:
> On 2/25/2020 4:58 PM, Jacob Keller wrote:
> > On 2/25/2020 4:42 PM, Sean Christopherson wrote>> 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.
> >>
> >
> > If you have any further suggestions for debugging, I'm happy to help try
> > to figure this out. To my eyes, it looks like somehow bits get reset...
> > It's definitely not clear to me how this happens.
> >
> > There is the get_cpu_caps call.. but that seems to correctly call
> > apply_forced_caps at the end.
> >
> > That's all I have time for today.
> >
> > Thanks,
> > Jake
> >
>
> Hi,
>
> I kept digging into this, and I added a further print to the get_cpu_cap
> function.
>
> It looks like get_cpu_cap is being called again *after*
> init_ia32_feat_ctl...
>
> Digging further, I discovered this appears to be the call in setup_pku,
> which would only be enabled for systems which have X86_FEATURE_PKU
> enabled and supported. It's quite likely that test systems may not have
> had this feature, hence why it went undetected till now.

Ya, probably not a whole lot of folks with Icelake silicon and VMX disabled
in BIOS. I'll see if I can reproduce on my ICX system, that would make
testing a fix a little easier.

> Because of the extra get_cpu_cap call, the capabilities are reset. Since
> we never use setup_clear_cpu_cap or pass NULL to clear_cpu_cap, the code
> that sets the global cpu_caps_cleared bits is not run.
>
> It's not clear to me what the best fix for this is.
>
> Perhaps init_ia32_feat_ctl should be something run during
> early_identify_cpu, since it's really checking global status (rdmsr),
> and not per-CPU status. And then it could directly operate to call
> setup_clear_cpu_cap, which would properly clear the bit globally,
> ensuring that apply_forced_caps kicks in?
>
> Or this needs to somehow be run *after* setup_pku? But that doesn't feel
> very robust.

Bummer. Using clear_cpu_cap() instead of setup_clear_cpu_cap() was me
being fancy and trying to allow KVM to identify the case where VMX is
available and configured on some CPUs but not all. I'll work on a fix.