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 10:27:31 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCHV2 3/3] x86, ras: Add mcsafe_memcpy() function to recover
 from machine checks

On Tue, Dec 15, 2015 at 9:53 AM, Luck, Tony <tony.luck@...el.com> wrote:
>>> ... 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...?
>
> My current generation cpu has a bit of an issue with recovering from a
> machine check in a "rep mov" ... so I'm working with a version of memcpy
> that unrolls into individual mov instructions for now.
>
>> At least the pmem driver use case does not want caching of the
>> source-buffer since that is the raw "disk" media.  I.e. in
>> pmem_do_bvec() we'd use this to implement memcpy_from_pmem().
>> However, caching the destination-buffer may prove beneficial since
>> that data is likely to be consumed immediately by the thread that
>> submitted the i/o.
>
> I can drop the "nti" from the destination moves.  Does "nti" work
> on the load from source address side to avoid cache allocation?

My mistake, I don't think we have an uncached load capability, only store.

> On another topic raised by Boris ... is there some CONFIG_PMEM*
> that I should use as a dependency to enable all this?

I'd rather make this a "select ARCH_MCSAFE_MEMCPY".  Since it's not a
hard dependency and the details will be hidden behind
memcpy_from_pmem().  Specifically, the details will be handled by a
new arch_memcpy_from_pmem() in arch/x86/include/asm/pmem.h to
supplement the existing arch_memcpy_to_pmem().
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