linux-kernel - Re: [PATCHV2 3/3] x86, ras: Add mcsafe_memcpy() function to recover from machine checks

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Date:	Tue, 15 Dec 2015 14:11:35 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Tony Luck <tony.luck@...el.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-nvdimm@...1.01.org, x86@...nel.org
Subject: Re: [PATCHV2 3/3] x86, ras: Add mcsafe_memcpy() function to recover
 from machine checks

On Thu, Dec 10, 2015 at 04:21:50PM -0800, Tony Luck wrote:
> Using __copy_user_nocache() as inspiration create a memory copy
> routine for use by kernel code with annotations to allow for
> recovery from machine checks.
> 
> Notes:
> 1) Unlike the original we make no attempt to copy all the bytes
>    up to the faulting address. The original achieves that by
>    re-executing the failing part as a byte-by-byte copy,
>    which will take another page fault. We don't want to have
>    a second machine check!
> 2) Likewise the return value for the original indicates exactly
>    how many bytes were not copied. Instead we provide the physical
>    address of the fault (thanks to help from do_machine_check()
> 3) Provide helpful macros to decode the return value.
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  arch/x86/include/asm/uaccess_64.h |  5 +++
>  arch/x86/kernel/x8664_ksyms_64.c  |  2 +
>  arch/x86/lib/copy_user_64.S       | 91 +++++++++++++++++++++++++++++++++++++++
>  3 files changed, 98 insertions(+)

...

> + * mcsafe_memcpy - Uncached memory copy with machine check exception handling
> + * Note that we only catch machine checks when reading the source addresses.
> + * Writes to target are posted and don't generate machine checks.
> + * This will force destination/source out of cache for more performance.

... and the non-temporal version is the optimal one even though we're
defaulting to copy_user_enhanced_fast_string for memcpy on modern Intel
CPUs...?

Btw, it should be also inside an ifdef if we're going to ifdef
CONFIG_MCE_KERNEL_RECOVERY everywhere else.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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