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Date: Wed, 6 Oct 2021 18:20:41 +0200 From: Borislav Petkov <bp@...en8.de> To: "Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@...ux.intel.com> Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>, David Hildenbrand <david@...hat.com>, Andrea Arcangeli <aarcange@...hat.com>, Josh Poimboeuf <jpoimboe@...hat.com>, Juergen Gross <jgross@...e.com>, Deep Shah <sdeep@...are.com>, VMware Inc <pv-drivers@...are.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>, Peter H Anvin <hpa@...or.com>, Dave Hansen <dave.hansen@...el.com>, Tony Luck <tony.luck@...el.com>, Dan Williams <dan.j.williams@...el.com>, Andi Kleen <ak@...ux.intel.com>, Kirill Shutemov <kirill.shutemov@...ux.intel.com>, Sean Christopherson <seanjc@...gle.com>, Kuppuswamy Sathyanarayanan <knsathya@...nel.org>, linux-kernel@...r.kernel.org Subject: Re: [PATCH v8 03/11] x86/cpufeatures: Add TDX Guest CPU feature 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
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