linux-kernel - Re: [PATCH v2] x86/mce: Reduce number of machine checks taken during recovery

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Message-ID: <YcWz6aEfsOZlHHXp@hirez.programming.kicks-ass.net>
Date:   Fri, 24 Dec 2021 12:50:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        Youquan Song <youquan.song@...el.com>
Subject: Re: [PATCH v2] x86/mce: Reduce number of machine checks taken during
 recovery

On Thu, Dec 23, 2021 at 12:07:01PM -0800, Luck, Tony wrote:
> diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S
> index e6ac38587b40..26781cbe7e37 100644
> --- a/arch/x86/lib/copy_user_64.S
> +++ b/arch/x86/lib/copy_user_64.S
> @@ -212,6 +212,7 @@ EXPORT_SYMBOL(copy_user_enhanced_fast_string)
>   * Don't try to copy the tail if machine check happened
>   *
>   * Input:
> + * eax x86 trap number - set by fixup_excpetion()

That's inaccurate, fixup_exception() (event if it's spelled correctly)
does not unconditionally set the trap number in RAX, that's only done by
ex_handler_fault() (or ex_handler_sgx()), which means all flows into
this function must pass through: EX_TYPE_FAULT, EX_TYPE_FAULT_MCE or
EX_TYPE_COPY.

Boris might fix up your comment if he applies I suppose..

>   * rdi destination
>   * rsi source
>   * rdx count
> @@ -220,12 +221,20 @@ EXPORT_SYMBOL(copy_user_enhanced_fast_string)
>   * eax uncopied bytes or 0 if successful.
>   */
>  SYM_CODE_START_LOCAL(.Lcopy_user_handle_tail)
> +	cmp $X86_TRAP_MC,%eax
> +	je 3f
> +
>  	movl %edx,%ecx
>  1:	rep movsb
>  2:	mov %ecx,%eax
>  	ASM_CLAC
>  	RET
>  
> +3:
> +	movl %edx,%eax
> +	ASM_CLAC
> +	RET
> +
>  	_ASM_EXTABLE_CPY(1b, 2b)
>  
>  .Lcopy_user_handle_align:
> 
> base-commit: 82a8954acd93ae95d6252fb93a3d210c8f71b093
> -- 
> 2.31.1
>