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Date:   Wed, 6 Oct 2021 18:20:41 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>, Deep Shah <sdeep@vmware.com>,
        VMware Inc <pv-drivers@vmware.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, Peter H Anvin <hpa@zytor.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 03/11] x86/cpufeatures: Add TDX Guest CPU feature
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On Wed, Oct 06, 2021 at 08:43:48AM -0700, Kuppuswamy, Sathyanarayanan wrote:
> 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().

That would normally make sense if this were a fast path. But I don't see
that being one. Besides, the subsequent runs will be the function call +
a couple of instructions:

# arch/x86/kernel/tdx.c:22: 	if (tdx_guest >= 0)
	movl	tdx_guest(%rip), %eax	# tdx_guest,
	testl	%eax, %eax	#
	jns	.L11	#,

...

.L11:
# arch/x86/kernel/tdx.c:23: 		return tdx_guest;
	setne	%al	#, <retval>
# arch/x86/kernel/tdx.c:35: }
	ret

which is not the end of the world.

And it if it is really used in a fast path, then we can do a static
branch etc.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette