linux-kernel - Re: [PATCH v3] x86/asm: Force native_apic_mem

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Date: Thu, 8 Feb 2024 08:47:46 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Adam Dunlap <acdunlap@...gle.com>, 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>, Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Arjan van de Ven <arjan@...ux.intel.com>, Wei Liu <wei.liu@...nel.org>,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
 Jacob Xu <jacobhxu@...gle.com>, Alper Gun <alpergun@...gle.com>,
 Kevin Loughlin <kevinloughlin@...gle.com>, Peter Gonda <pgonda@...gle.com>,
 Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v3] x86/asm: Force native_apic_mem_read to use mov

On 2/6/24 14:36, Adam Dunlap wrote:
..
> In particular, when compiled with clang and run as a SEV-ES or
> SEV-SNP guest, the compiler would emit a testl instruction which is 
> not supported by the SEV-ES emulator

What changed?  Why is this a bug that we're only noticing now?  The line
of code that's modified here is from 2008.

I assume that it's something new in clang, but it'd be great to know
that for sure.

Also, considering the age of the last commit to touch that line:

Fixes: 67c5fc5c330f ("x86: merge apic_32/64.h")

this seems like the kind of thing we'll want in -stable in case folks
are compiling stable kernels with new clangs.