linux-kernel - Re: [PATCH v11 3/4] x86, mce: Add __mcsafe

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Date:	Thu, 18 Feb 2016 10:12:42 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tony Luck <tony.luck@...el.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v11 3/4] x86, mce: Add __mcsafe_copy()

On Wed, Feb 17, 2016 at 10:20 AM, Tony Luck <tony.luck@...el.com> wrote:
>
> If we faulted during the copy, then 'trapnr' will say which type
> of trap (X86_TRAP_PF or X86_TRAP_MC) and 'remain' says how many
> bytes were not copied.

So apart from the naming, a couple of questions:

 - I'd like to see the actual *use* case explained, not just what it does.

 - why does this use the complex - and slower, on modern machines -
unrolled manual memory copy, when you might as well just use a single

     rep ; movsb

    which not only makes it smaller, but makes the exception fixup trivial.

 - why not make the "bytes remaining" the same as for a user-space
copy (ie return it as the return value)?

 - at that point, it ends up looking a *lot* like uaccess_try/catch,
which gets the error code from current_thread_info()->uaccess_err

Hmm?

          Linus