Re: [PATCH v10 03/11] x86/cpufeatures: Add TDX Guest CPU feature

[Sathyanarayanan Kuppuswamy]

On 10/13/21 1:44 PM, Thomas Gleixner wrote:
> On Fri, Oct 08 2021 at 22:37, Kuppuswamy Sathyanarayanan wrote:
> > +
> > +bool is_tdx_guest(void)
> > +{
> > +	static int tdx_guest = -1;
> > +	u32 eax, sig[3];
> > +
> > +	if (tdx_guest >= 0)
> > +		goto done;
> > +
> > +	if (cpuid_eax(0) < TDX_CPUID_LEAF_ID) {
> > +		tdx_guest = 0;
> > +		goto done;
> > +	}
> > +
> > +	cpuid_count(TDX_CPUID_LEAF_ID, 0, &eax, &sig[0], &sig[2], &sig[1]);
> > +
> > +	tdx_guest = !memcmp("IntelTDX    ", sig, 12);
> > +
> > +done:
> > +	return !!tdx_guest;
> > +}
> No. This is tasteless garbage, really.
>
> tdx_early_init() is invoked from x86_64_start_kernel() very early in the
> boot process __before__ is_tdx_guest() is invoked.
>
> So why on earth is it requried to keep those conditionals and cpuid()
> muck around after init?

is_tdx_guest() is also used by cc_platform_has() API. Please check
the following patch [1]. cc_platform_has() will be called much earlier
than x86_64_start_kernel() (like __startup_64() [2]).

[1] - 
https://lore.kernel.org/lkml/20211009053747.1694419-5-sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx/
[2] - 
https://lore.kernel.org/lkml/46a18427dc4e9dda985b10e472965e3e4c769f1d.1631141919.git.thomas.lendacky@xxxxxxx/

> > +void __init tdx_early_init(void)
> > +{
> > +	if (!is_tdx_guest())
> > +		return;
> > +
> > +	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
> > +
> > +	pr_info("Guest initialized\n");
> > +}
> What's wrong with:
>
> static bool tdx_guest_detected __ro_after_init;
>
> void __init tdx_early_init(void)
> {
> 	u32 eax, sig[3];
>
> 	if (cpuid_eax(0) < TDX_CPUID_LEAF_ID)
>          	return;
>
>          cpuid_count(TDX_CPUID_LEAF_ID, 0, &eax, &sig[0], &sig[2],  &sig[1]);
>
>          if (memcmp("IntelTDX    ", sig, 12))
>          	return;
>
>          tdx_guest_detected = true;
> 	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
>
> 	pr_info("Guest initialized\n");
> }
>
> bool is_tdx_guest(void)
> {
>          return tdx_guest_detected;
> }
>
> That's simple and obvious and discards all the detection gunk completely
> after init and uses the proper data type, right?
>
> Thanks,
>
>          tglx

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer