linux-kernel - Re: [PATCH v2 1/6] x86/bugs: Add asm helpers for executing VERW

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Date:   Wed, 25 Oct 2023 08:58:18 +0200
From:   Peter Zijlstra <peterz@...radead.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>,
        Josh Poimboeuf <jpoimboe@...nel.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,
        Alyssa Milburn <alyssa.milburn@...el.com>
Subject: Re: [PATCH  v2 1/6] x86/bugs: Add asm helpers for executing VERW

On Tue, Oct 24, 2023 at 09:00:29PM -0700, Pawan Gupta wrote:

> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index c55cc243592e..ed8218e2d9a7 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -13,6 +13,7 @@
>  #include <asm/unwind_hints.h>
>  #include <asm/percpu.h>
>  #include <asm/current.h>
> +#include <asm/segment.h>
>  
>  /*
>   * Call depth tracking for Intel SKL CPUs to address the RSB underflow
> @@ -329,6 +330,29 @@
>  #endif
>  .endm
>  
> +/*
> + * Macros to execute VERW instruction that mitigate transient data sampling
> + * attacks such as MDS. On affected systems a microcode update overloaded VERW
> + * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
> + *
> + * Note: Only the memory operand variant of VERW clears the CPU buffers.
> + */
> +.pushsection .rodata
> +.align 64
> +mds_verw_sel:
> +	.word __KERNEL_DS
> + 	.byte 0xcc
> +.align 64
> +.popsection

This should not be in a header file, you'll get an instance of this per
translation unit, not what you want.

> +
> +.macro EXEC_VERW
> +	verw _ASM_RIP(mds_verw_sel)
> +.endm
> +
> +.macro CLEAR_CPU_BUFFERS
> +	ALTERNATIVE "", __stringify(EXEC_VERW), X86_FEATURE_CLEAR_CPU_BUF
> +.endm
> +
>  #else /* __ASSEMBLY__ */