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Date: Wed, 10 Jan 2024 09:49:44 -0800
From: Andi Kleen <ak@linux.intel.com>
To: Kevin Loughlin <kevinloughlin@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Kees Cook <keescook@chromium.org>,
	"Masami Hiramatsu (Google)" <mhiramat@kernel.org>,
	Ze Gao <zegao2021@gmail.com>, Josh Poimboeuf <jpoimboe@kernel.org>,
	Pengfei Xu <pengfei.xu@intel.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Michael Roth <michael.roth@amd.com>,
	Ashish Kalra <ashish.kalra@amd.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Joerg Roedel <jroedel@suse.de>, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev, linux-coco@lists.linux.dev,
	Adam Dunlap <acdunlap@google.com>, Peter Gonda <pgonda@google.com>,
	Jacob Xu <jacobhxu@google.com>,
	Sidharth Telang <sidtelang@google.com>
Subject: Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in
 clang
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References: <20240110012640.1335694-1-kevinloughlin@google.com>
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> On that note, I do have another version of this patch that abstracts
> snp_cpuid_get_table() into a macro along the lines of...
> 
> #define GET_RIP_RELATIVE_PTR(var) \
> ({  \
>  void *ptr; \
>  asm ("lea "#var"(%%rip), %0" \
>       : "=r" (ptr) \
>       : "p" (&var)); \
>  ptr; \
> })
> 
> ...and uses this new macro to access all SEV/SME global variables (not
> just the cpuid_table). It's similar in nature to `fixup_pointer()`
> (currently defined in arch/x86/kernel/head64.c) but doesn't require us
> to pass around `physaddr` from `__startup64()`. This wouldn't
> introduce any new execution model changes between clang vs gcc and
> would be consistent with the kernel's current approach of relying on
> developers to manually apply fixups for global variable accesses prior
> to kernel relocation. I can send an RFC v2 for the
> GET_RIP_RELATIVE_PTR() version of this patch.

That looks like a far better solution indeed.

Ideally objtool would check for this, perhaps with a new ELF
section. But actually doing that might be far more work, so perhaps not
worth it.

Thanks,

-Andi