linux-kernel - Re: [PATCH] x86/retpoline: Don't clobber RFLAGS during srso_safe

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Message-ID: <20230811180528.GJZNZ4aIHCn3zMaida@fat_crate.local>
Date:   Fri, 11 Aug 2023 20:05:28 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Srikanth Aithal <sraithal@....com>,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] x86/retpoline: Don't clobber RFLAGS during
 srso_safe_ret()

On Fri, Aug 11, 2023 at 08:52:55AM -0700, Sean Christopherson wrote:
> A major motivation for doing fast emulation is to leverage the CPU to
> handle consumption and manipulation of arithmetic flags, i.e. RFLAGS is
> both an input and output to the target of the call.  fastop() collects
> the RFLAGS result by pushing RFLAGS onto the stack and popping them back
> into a variable (held in RDI in this case)
> 
>   asm("push %[flags]; popf; " CALL_NOSPEC " ; pushf; pop %[flags]\n"

Right, and I've tested this countless times with gcc-built host and
guest.

But Nathan's case where the host is built with gcc but the guest with
clang, would trigger this. And as he confirms, that fixes it so I wonder
what is the difference in code generation to make this rFLAGS corruption
noticeable in that particular configuration.

Oh well, later when the fires are put out.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette