linux-kernel - Re: [PATCH v4 4/6] x86/bugs: Use ALTERNATIVE() instead of mds_user

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Date:   Fri, 1 Dec 2023 11:59:54 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Nikolay Borisov <nik.borisov@...e.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kvm@...r.kernel.org,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        antonio.gomez.iglesias@...ux.intel.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH  v4 4/6] x86/bugs: Use ALTERNATIVE() instead of
 mds_user_clear static key

On Fri, Oct 27, 2023 at 07:38:59AM -0700, Pawan Gupta wrote:
> The VERW mitigation at exit-to-user is enabled via a static branch
> mds_user_clear. This static branch is never toggled after boot, and can
> be safely replaced with an ALTERNATIVE() which is convenient to use in
> asm.
> 
> Switch to ALTERNATIVE() to use the VERW mitigation late in exit-to-user
> path. Also remove the now redundant VERW in exc_nmi() and
> arch_exit_to_user_mode().
> 
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> ---
>  Documentation/arch/x86/mds.rst       | 38 +++++++++++++++++++++++++-----------
>  arch/x86/include/asm/entry-common.h  |  1 -
>  arch/x86/include/asm/nospec-branch.h | 12 ------------
>  arch/x86/kernel/cpu/bugs.c           | 15 ++++++--------
>  arch/x86/kernel/nmi.c                |  2 --
>  arch/x86/kvm/vmx/vmx.c               |  2 +-
>  6 files changed, 34 insertions(+), 36 deletions(-)
> 
> diff --git a/Documentation/arch/x86/mds.rst b/Documentation/arch/x86/mds.rst
> index e73fdff62c0a..a5c5091b9ccd 100644
> --- a/Documentation/arch/x86/mds.rst
> +++ b/Documentation/arch/x86/mds.rst
> @@ -95,6 +95,9 @@ The kernel provides a function to invoke the buffer clearing:
>  
>      mds_clear_cpu_buffers()
>  
> +Also macro CLEAR_CPU_BUFFERS is meant to be used in ASM late in exit-to-user
> +path. This macro works for cases where GPRs can't be clobbered.

What does this last sentence mean?  Is it trying to say that the macro
doesn't clobber registers (other than ZF)?

-- 
Josh