linux-kernel - Re: Hang when booting guest kernels compiled with clang after SRSO mitigations

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Date:   Thu, 10 Aug 2023 14:51:22 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: Hang when booting guest kernels compiled with clang after SRSO
 mitigations

On Thu, Aug 10, 2023 at 03:16:49AM -0700, Nathan Chancellor wrote:
> Just to clarify, this is the guest kernel at -rc5 and the host kernel
> with the SRSO mitigations applied? If so, that's the problem. The guest
> kernel does not have to have the SRSO mitigations applied to see this
> problem. Sorry I should have made that more clear! If not though, that's
> interesting because I was running -rc5 on the host without issues.

Well, how do you even build CPU_SRSO with clang?

config CPU_SRSO
        bool "Mitigate speculative RAS overflow on AMD"
        depends on CPU_SUP_AMD && X86_64 && RETHUNK
					    ^^^^^^^

config RETHUNK
        bool "Enable return-thunks"
        depends on RETPOLINE && CC_HAS_RETURN_THUNK
				^^^^^^^^^^^^^^^^^^^

config CC_HAS_RETURN_THUNK
        def_bool $(cc-option,-mfunction-return=thunk-extern)

$ clang -mfunction-return=thunk-extern
clang: error: unknown argument: '-mfunction-return=thunk-extern'
clang: error: no input files

$ clang --version
Debian clang version 14.0.6
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Hmmm.

-- 
Regards/Gruss,
    Boris.

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