linux-kernel - Re: [PATCH 2/6] x86/entry_64: Add VERW just before userspace transition

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Date:   Mon, 23 Oct 2023 11:22:11 -0700
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,
        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,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH  2/6] x86/entry_64: Add VERW just before userspace
 transition

On Fri, Oct 20, 2023 at 01:45:03PM -0700, Pawan Gupta wrote:
> +	/* Mitigate CPU data sampling attacks .e.g. MDS */
> +	USER_CLEAR_CPU_BUFFERS
> +
>  	jmp	.Lnative_iret
>  
>  
> @@ -774,6 +780,9 @@ native_irq_return_ldt:
>  	 */
>  	popq	%rax				/* Restore user RAX */
>  
> +	/* Mitigate CPU data sampling attacks .e.g. MDS */
> +	USER_CLEAR_CPU_BUFFERS
> +

I'm thinking the comments add unnecessary noise here.  The macro name is
self-documenting enough.

The detail about what mitigations are being done can go above the macro
definition itself, which the reader can refer to if they want more
detail about what the macro is doing and why.

Speaking of the macro name, I think just "CLEAR_CPU_BUFFERS" is
sufficient.  The "USER_" prefix makes it harder to read IMO.

-- 
Josh