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Date: Wed, 10 Jan 2024 03:45:13 -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>,
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	Justin Stitt <justinstitt@google.com>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Kees Cook <keescook@chromium.org>,
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	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
Message-ID: <ZZ6DSdaaoYKzgp4K@tassilo>
References: <20240110012640.1335694-1-kevinloughlin@google.com>
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In-Reply-To: <20240110012640.1335694-1-kevinloughlin@google.com>

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