linux-kernel - Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in clang

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Date: Wed, 10 Jan 2024 03:45:13 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: Kevin Loughlin <kevinloughlin@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Kees Cook <keescook@...omium.org>,
	"Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
	Ze Gao <zegao2021@...il.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
	Pengfei Xu <pengfei.xu@...el.com>,
	Brijesh Singh <brijesh.singh@....com>,
	Michael Roth <michael.roth@....com>,
	Ashish Kalra <ashish.kalra@....com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, linux-coco@...ts.linux.dev,
	Adam Dunlap <acdunlap@...gle.com>, Peter Gonda <pgonda@...gle.com>,
	Jacob Xu <jacobhxu@...gle.com>,
	Sidharth Telang <sidtelang@...gle.com>
Subject: Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in
 clang

On Wed, Jan 10, 2024 at 01:26:39AM +0000, Kevin Loughlin wrote:
> SEV/SME code can execute prior to page table fixups for kernel

That seems to be fundamental problem.

> relocation. However, as with global variables accessed in
> __startup_64(), clang does not currently generate PC-relative accesses
> for SEV/SME global variables, causing certain flavors of SEV hosts and
> guests to crash.

And that's a hack to work around it.

> 
> While an attempt was made to force PC-relative addressing for certain
> global SEV/SME variables via inline assembly (see snp_cpuid_get_table()
> for example), PC-relative addressing must be pervasively-enforced for
> SEV/SME global variables that can be accessed prior to page table
> fixups.
> 
> To avoid the error-prone approach of manually referencing each SEV/SME
> global variable via a general form of snp_cpuid_get_table(), it is
> preferable to use compiler flags for position-independent code (ex:

But if gcc doesn't support it then it doesn't work.

It seems your approach with incompatible execution models between
the compilers is just a recipe for future patches only working
on one of the compilers because most code submitters probably
won't test both.

It would be better to at least use a unified execution model, if you want
to extend the hack and not fix the underlying issue.

-Andi