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

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Message-ID: <20160218185116.GB16753@gmail.com>
Date:	Thu, 18 Feb 2016 19:51:16 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Tony Luck <tony.luck@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v11 3/4] x86, mce: Add __mcsafe_copy()


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 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?

memcpy_try()/memcpy_catch() definitely has a nice ring to it.

Thanks,

	Ingo