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From: Ard Biesheuvel <ardb@kernel.org>
Date: Fri, 9 Feb 2024 15:22:18 +0000
Message-ID: <CAMj1kXE4Yhh_5y=ybFkD7YQUZuh7+-P_Vyzv-gpvbQ=i=eRMnw@mail.gmail.com>
Subject: Re: [PATCH v3] x86/asm: Force native_apic_mem_read to use mov
To: Dave Hansen <dave.hansen@intel.com>
Cc: Adam Dunlap <acdunlap@google.com>, 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>, Nathan Chancellor <nathan@kernel.org>, 
	Nick Desaulniers <ndesaulniers@google.com>, Bill Wendling <morbo@google.com>, 
	Justin Stitt <justinstitt@google.com>, "Peter Zijlstra (Intel)" <peterz@infradead.org>, 
	Arjan van de Ven <arjan@linux.intel.com>, Wei Liu <wei.liu@kernel.org>, linux-kernel@vger.kernel.org, 
	llvm@lists.linux.dev, Jacob Xu <jacobhxu@google.com>, Alper Gun <alpergun@google.com>, 
	Kevin Loughlin <kevinloughlin@google.com>, Peter Gonda <pgonda@google.com>
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On Thu, 8 Feb 2024 at 16:48, Dave Hansen <dave.hansen@intel.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.