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

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Date: Fri, 9 Feb 2024 15:22:18 +0000
From: Ard Biesheuvel <ardb@...nel.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: 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>
Subject: Re: [PATCH v3] x86/asm: Force native_apic_mem_read to use mov

On Thu, 8 Feb 2024 at 16:48, Dave Hansen <dave.hansen@...el.com> wrote:
>
> 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.
>

Might be the use of LTO in the Google prod[uction]kernel. Adam, can you confirm?

> 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.

LTO support was introduced in v5.12 afaict.