Re: [PATCHv2 01/29] x86/tdx: Detect running as a TDX guest in early boot

Josh Poimboeuf
2022-02-03T00:32:20Z

On Wed, Feb 02, 2022 at 02:14:59AM +0300, Kirill A. Shutemov wrote:
> On Tue, Feb 01, 2022 at 08:29:55PM +0100, Thomas Gleixner wrote:
> > Kirill,
> >
> > On Mon, Jan 24 2022 at 18:01, Kirill A. Shutemov wrote:
> >
> > Just a nitpick...
> >
> > > +static bool tdx_guest_detected __ro_after_init;
> > > +
> > > +bool is_tdx_guest(void)
> > > +{
> > > + return tdx_guest_detected;
> > > +}
> > > +
> > > +void __init tdx_early_init(void)
> > > +{
> > > + u32 eax, sig[3];
> > > +
> > > + cpuid_count(TDX_CPUID_LEAF_ID, 0, &eax, &sig[0], &sig[2], &sig[1]);
> > > +
> > > + if (memcmp(TDX_IDENT, sig, 12))
> > > + return;
> > > +
> > > + tdx_guest_detected = true;
> > > +
> > > + setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
> >
> > So with that we have two ways to detect a TDX guest:
> >
> > - tdx_guest_detected
> >
> > - X86_FEATURE_TDX_GUEST
> >
> > Shouldn't X86_FEATURE_TDX_GUEST be good enough?
>
> Right. We have only 3 callers of is_tdx_guest() in cc_platform.c
> I will replace them with cpu_feature_enabled(X86_FEATURE_TDX_GUEST).

I had the same review comment before. I was told that cc_platform_has()
was called before caps have been set up properly, so caps can't be
relied upon this early.

I'm not really convinced that's true. Yes, early_identify_cpu() --
which runs after tdx_early_init() -- clears all boot_cpu_data's
capability bits to zero [*].

But shortly after that, early_identify_cpu() restores any "forced" caps
with a call to get_cpu_cap() -> apply_forced_caps().

So as far as I can tell, while it's subtle, it should work. However, it
should be tested carefully ;-)

[ *] The memset() of boot_cpu_data seems unnecessary since it should
already be cleared by the compiler when it gets placed in the
.data..read_mostly section.

--
Josh