Re: [PATCH v8 03/11] x86/cpufeatures: Add TDX Guest CPU feature

[Kuppuswamy, Sathyanarayanan]

On 10/6/21 8:26 AM, Borislav Petkov wrote:
> On Mon, Oct 04, 2021 at 07:51:57PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
> > new file mode 100644
> > index 000000000000..ad3ff5925153
> > --- /dev/null
> > +++ b/arch/x86/kernel/tdx.c
> > @@ -0,0 +1,40 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/* Copyright (C) 2020 Intel Corporation */
> > +
> > +#undef pr_fmt
> > +#define pr_fmt(fmt)     "tdx: " fmt
> > +
> > +#include <asm/tdx.h>
> > +
> > +/*
> > + * Allocate it in the data region to avoid zeroing it during
> > + * BSS initialization. It is mainly used in cc_platform_has()
> > + * call during early boot call.
> > + */
> > +u64 __section(".data") is_tdx_guest = 0;
> > +
> > +static void __init is_tdx_guest_init(void)
> > +{
> > +	u32 eax, sig[3];
> > +
> > +	if (cpuid_eax(0) < TDX_CPUID_LEAF_ID) {
> > +		is_tdx_guest = 0;
> > +		return;
> > +	}
> > +
> > +	cpuid_count(TDX_CPUID_LEAF_ID, 0, &eax, &sig[0], &sig[2], &sig[1]);
> > +
> > +	is_tdx_guest = !memcmp("IntelTDX    ", sig, 12);
> > +}
> > +
> > +void __init tdx_early_init(void)
> > +{
> > +	is_tdx_guest_init();
> > +
> > +	if (!is_tdx_guest)
> > +		return;
> > +
> > +	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
> > +
> > +	pr_info("Guest initialized\n");
> > +}
> > --
>
> What I meant was this (untested of course).
>
> is_tdx_guest() is the accessor external code queries and you cache the
> detected value in tdx_guest so that the one after the first one is
> cheap.

Yes. But, Joerg Roedel in his review recommended using variable
similar to sme_me_mask to avoid function call in Intel platform in
cc_platform_has().

"
This causes a function call on every Intel machine this code runs. is
there an easier to check whether TDX is enabled, like the sme_me_mask
check on AMD?
"

That's why I have introduced is_tdx_guest global variable in this
version.


> /*
>   * Allocate it in the data region to avoid zeroing it during
>   * BSS initialization. It is mainly used in cc_platform_has()
>   * call during early boot call.
>   *
>   * States whether the kernel is running as a TDX guest.
>   */
> static int tdx_guest __ro_after_init = -1;
>
> bool is_tdx_guest(void)
> {
> 	u32 eax, sig[3];
>
> 	if (tdx_guest >= 0)
> 		return tdx_guest;
>
> 	if (cpuid_eax(0) < TDX_CPUID_LEAF_ID) {
> 		tdx_guest = 0;
> 		return false;
> 	}
>
> 	cpuid_count(TDX_CPUID_LEAF_ID, 0, &eax, &sig[0], &sig[2], &sig[1]);
>
> 	tdx_guest = !memcmp("IntelTDX    ", sig, 12);
>
> 	return tdx_guest;
> }
>
> void __init tdx_early_init(void)
> {
> 	if (!is_tdx_guest())
> 		return;
>
> 	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
>
> 	pr_info("Guest initialized\n");
> }

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer