Subject: Re: [PATCH v14] x86, mce: Add memcpy_mcsafe()
To: Tony Luck <tony.luck@intel.com>, Ingo Molnar <mingo@kernel.org>
References: <a44e1055efc2d2a9473307b22c91caa437aa3f8b.1456439214.git.tony.luck@intel.com>
CC: <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tony Luck <tony.luck@gmail.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>
From: =?UTF-8?Q?Mika_Penttil=c3=a4?= <mika.penttila@nextfour.com>
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Date: Thu, 10 Mar 2016 21:26:51 +0200
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On 18.02.2016 21:47, Tony Luck wrote:
> Make use of the EXTABLE_FAULT exception table entries to write
> a kernel copy routine that doesn't crash the system if it
> encounters a machine check. Prime use case for this is to copy
> from large arrays of non-volatile memory used as storage.
>
> We have to use an unrolled copy loop for now because current
> hardware implementations treat a machine check in "rep mov"
> as fatal. When that is fixed we can simplify.
>
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> ---
>
> Is this what we want now?  Return type is a "bool". True means
> that we copied OK, false means that it didn't (this is all that
> Dan says that he needs).  Dropped all the complex code to figure
> out how many bytes we didn't copy as Linus says this isn't the
> right place to do this (and besides we should just make "rep mov"
>
> +
> +	/* Copy successful. Return true */
> +.L_done_memcpy_trap:
> +	xorq %rax, %rax
> +	ret
> +ENDPROC(memcpy_mcsafe)
> +
> +	.section .fixup, "ax"
> +	/* Return false for any failure */
> +.L_memcpy_mcsafe_fail:
> +	mov	$1, %rax
> +	ret
> +
>
But you return 0 == false for success and 1 == true for failure.

--Mika