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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Tony Luck <tony.luck@intel.com>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
	Peter Zijlstra <peterz@infradead.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter()
Date: Wed, 02 May 2018 04:14:41 +0000	[thread overview]
Message-ID: <CA+55aFwjaY6Nv7x0d8BQaSddZwAK7fukrYOHWEG_v2-1JZi5Qg@mail.gmail.com> (raw)
In-Reply-To: <CAPcyv4idV+hp-W56gyQDN4p9SQsYz+xondgVJwQSYphUMxkYnw@mail.gmail.com>

On Tue, May 1, 2018 at 9:00 PM Dan Williams <dan.j.williams@intel.com>
wrote:
> >
> > I  have some dim memory of "rep movs doesn't work well for pmem", but
does
> > it *seriously* need unrolling to cacheline boundaries? And if it does,
who
> > designed it, and why is anybody using it?
> >

> I think this is an FAQ from the original submission, in fact some guy
> named "Linus Torvalds" asked [1]:

Oh, I already mentioned that  I remembered that "rep movs" didn't work well.

But there's a big gap between "just use 'rep movs' and 'do some cacheline
unrollong'".

Why isn't it just doing a simple word-at-a-time loop and letting the CPU do
the unrolling that it will already do on its own?

I may have gotten that answered too, but there's no comment in the code
about why it's such a disgusting mess, so I've long since forgotten _why_
it's such a disgusting mess.

That loop unrolling _used_ to be "hey, it's simple".

Now it's "Hey, that's truly disgusting", with the separate fault handling
for every single case in the unrolled loop.

Just look at the nasty _ASM_EXTABLE_FAULT() uses and those E_cache_x error
labels, and getting the number rof bytes copied right.

And then ask yourself "what if we didn't unroll that thing 8 times, AND WE
COULD GET RID OF ALL OF THOSE?"

Maybe you already did ask yourself.  But I'm asking because it sure isn't
explained in the code.

             Linus
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WARNING: multiple messages have this Message-ID (diff)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
	Tony Luck <tony.luck@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@amacapital.net>,
	Ingo Molnar <mingo@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter()
Date: Wed, 02 May 2018 04:14:41 +0000	[thread overview]
Message-ID: <CA+55aFwjaY6Nv7x0d8BQaSddZwAK7fukrYOHWEG_v2-1JZi5Qg@mail.gmail.com> (raw)
In-Reply-To: <CAPcyv4idV+hp-W56gyQDN4p9SQsYz+xondgVJwQSYphUMxkYnw@mail.gmail.com>

On Tue, May 1, 2018 at 9:00 PM Dan Williams <dan.j.williams@intel.com>
wrote:
> >
> > I  have some dim memory of "rep movs doesn't work well for pmem", but
does
> > it *seriously* need unrolling to cacheline boundaries? And if it does,
who
> > designed it, and why is anybody using it?
> >

> I think this is an FAQ from the original submission, in fact some guy
> named "Linus Torvalds" asked [1]:

Oh, I already mentioned that  I remembered that "rep movs" didn't work well.

But there's a big gap between "just use 'rep movs' and 'do some cacheline
unrollong'".

Why isn't it just doing a simple word-at-a-time loop and letting the CPU do
the unrolling that it will already do on its own?

I may have gotten that answered too, but there's no comment in the code
about why it's such a disgusting mess, so I've long since forgotten _why_
it's such a disgusting mess.

That loop unrolling _used_ to be "hey, it's simple".

Now it's "Hey, that's truly disgusting", with the separate fault handling
for every single case in the unrolled loop.

Just look at the nasty _ASM_EXTABLE_FAULT() uses and those E_cache_x error
labels, and getting the number rof bytes copied right.

And then ask yourself "what if we didn't unroll that thing 8 times, AND WE
COULD GET RID OF ALL OF THOSE?"

Maybe you already did ask yourself.  But I'm asking because it sure isn't
explained in the code.

             Linus

  reply	other threads:[~2018-05-02  4:14 UTC|newest]

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-01 20:45 [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter() Dan Williams
2018-05-01 20:45 ` Dan Williams
2018-05-01 20:45 ` [PATCH 1/6] x86, memcpy_mcsafe: update labels in support of write fault handling Dan Williams
2018-05-01 20:45   ` Dan Williams
2018-05-01 20:45 ` [PATCH 2/6] x86, memcpy_mcsafe: return bytes remaining Dan Williams
2018-05-01 20:45   ` Dan Williams
2018-05-01 20:45 ` [PATCH 3/6] x86, memcpy_mcsafe: add write-protection-fault handling Dan Williams
2018-05-01 20:45   ` Dan Williams
2018-05-01 20:45 ` [PATCH 4/6] x86, memcpy_mcsafe: define copy_to_iter_mcsafe() Dan Williams
2018-05-01 20:45   ` Dan Williams
2018-05-01 22:17   ` kbuild test robot
2018-05-01 22:17     ` kbuild test robot
2018-05-01 22:49   ` kbuild test robot
2018-05-01 22:49     ` kbuild test robot
2018-05-01 20:45 ` [PATCH 5/6] dax: use copy_to_iter_mcsafe() in dax_iomap_actor() Dan Williams
2018-05-01 20:45   ` Dan Williams
2018-05-01 20:45 ` [PATCH 6/6] x86, nfit_test: unit test for memcpy_mcsafe() Dan Williams
2018-05-01 20:45   ` Dan Williams
2018-05-01 21:05 ` [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter() Linus Torvalds
2018-05-01 21:05   ` Linus Torvalds
2018-05-01 23:02   ` Dan Williams
2018-05-01 23:02     ` Dan Williams
2018-05-01 23:28     ` Andy Lutomirski
2018-05-01 23:28       ` Andy Lutomirski
2018-05-01 23:31       ` Dan Williams
2018-05-01 23:31         ` Dan Williams
2018-05-02  0:09     ` Linus Torvalds
2018-05-02  0:09       ` Linus Torvalds
2018-05-02  2:25       ` Dan Williams
2018-05-02  2:25         ` Dan Williams
2018-05-02  2:53         ` Linus Torvalds
2018-05-02  2:53           ` Linus Torvalds
2018-05-02  3:02           ` Dan Williams
2018-05-02  3:02             ` Dan Williams
2018-05-02  3:13             ` Linus Torvalds
2018-05-02  3:13               ` Linus Torvalds
2018-05-02  3:20               ` Dan Williams
2018-05-02  3:20                 ` Dan Williams
2018-05-02  3:22                 ` Dan Williams
2018-05-02  3:22                   ` Dan Williams
2018-05-02  3:33                   ` Linus Torvalds
2018-05-02  3:33                     ` Linus Torvalds
2018-05-02  4:00                     ` Dan Williams
2018-05-02  4:00                       ` Dan Williams
2018-05-02  4:14                       ` Linus Torvalds [this message]
2018-05-02  4:14                         ` Linus Torvalds
2018-05-02  5:37                         ` Dan Williams
2018-05-02  5:37                           ` Dan Williams
2018-05-02 16:19                     ` Andy Lutomirski
2018-05-02 16:19                       ` Andy Lutomirski
2018-05-02 17:47                       ` Dan Williams
2018-05-02 17:47                         ` Dan Williams
2018-05-02  8:30         ` Borislav Petkov
2018-05-02  8:30           ` Borislav Petkov
2018-05-02 13:52           ` Dan Williams
2018-05-02 13:52             ` Dan Williams

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