From: David Laight <David.Laight@ACULAB•COM>
To: 'Ingo Molnar' <mingo@kernel•org>, Thomas Gleixner <tglx@linutronix•de>
Cc: 'Rahul Lakkireddy' <rahul.lakkireddy@chelsio•com>,
"x86@kernel•org" <x86@kernel•org>,
"linux-kernel@vger•kernel.org" <linux-kernel@vger•kernel.org>,
"netdev@vger•kernel.org" <netdev@vger•kernel.org>,
"mingo@redhat•com" <mingo@redhat•com>,
"hpa@zytor•com" <hpa@zytor•com>,
"davem@davemloft•net" <davem@davemloft•net>,
"akpm@linux-foundation•org" <akpm@linux-foundation•org>,
"torvalds@linux-foundation•org" <torvalds@linux-foundation•org>,
"ganeshgr@chelsio•com" <ganeshgr@chelsio•com>,
"nirranjan@chelsio•com" <nirranjan@chelsio•com>,
"indranil@chelsio•com" <indranil@chelsio•com>,
"Andy Lutomirski" <luto@kernel•org>,
Peter Zijlstra <a.p.zijlstra@chello•nl>,
Fenghua Yu <fenghua.yu@intel•com>,
Eric Biggers <ebiggers3@gmail•com>
Subject: RE: [RFC PATCH 0/3] kernel: add support for 256-bit IO access
Date: Tue, 20 Mar 2018 13:30:59 +0000 [thread overview]
Message-ID: <f3bfbb3d269f4dc29260bc8a6ae6ef22@AcuMS.aculab.com> (raw)
In-Reply-To: <20180320105427.bm4od7cpessbraag@gmail.com>
From: Ingo Molnar
> Sent: 20 March 2018 10:54
...
> Note that a generic version might still be worth trying out, if and only if it's
> safe to access those vector registers directly: modern x86 CPUs will do their
> non-constant memcpy()s via the common memcpy_erms() function - which could in
> theory be an easy common point to be (cpufeatures-) patched to an AVX2 variant, if
> size (and alignment, perhaps) is a multiple of 32 bytes or so.
>
> Assuming it's correct with arbitrary user-space FPU state and if it results in any
> measurable speedups, which might not be the case: ERMS is supposed to be very
> fast.
>
> So even if it's possible (which it might not be), it could end up being slower
> than the ERMS version.
Last I checked memcpy() was implemented as 'rep movsb' on the latest Intel cpus.
Since memcpy_to/fromio() get aliased to memcpy() this generates byte copies.
The previous 'fastest' version of memcpy() was ok for uncached locations.
For PCIe I suspect that the actual instructions don't make a massive difference.
I'm not even sure interleaving two transfers makes any difference.
What makes a huge difference for memcpy_fromio() is the size of the register.
The time taken for a read will be largely independent of the width of the
register used.
David
next prev parent reply other threads:[~2018-03-20 13:30 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-19 14:20 [RFC PATCH 0/3] kernel: add support for 256-bit IO access Rahul Lakkireddy
2018-03-19 14:20 ` [RFC PATCH 1/3] include/linux: add 256-bit IO accessors Rahul Lakkireddy
2018-03-19 14:20 ` [RFC PATCH 2/3] x86/io: implement 256-bit IO read and write Rahul Lakkireddy
2018-03-19 14:43 ` Thomas Gleixner
2018-03-20 13:32 ` Rahul Lakkireddy
2018-03-20 13:44 ` Andy Shevchenko
2018-03-21 12:27 ` Rahul Lakkireddy
2018-03-20 14:40 ` David Laight
2018-03-21 12:28 ` Rahul Lakkireddy
2018-03-20 14:42 ` Alexander Duyck
2018-03-21 12:28 ` Rahul Lakkireddy
2018-03-22 1:26 ` Linus Torvalds
2018-03-22 10:48 ` David Laight
2018-03-22 17:16 ` Linus Torvalds
2018-03-19 14:20 ` [RFC PATCH 3/3] cxgb4: read on-chip memory 256-bits at a time Rahul Lakkireddy
2018-03-19 14:53 ` [RFC PATCH 0/3] kernel: add support for 256-bit IO access David Laight
2018-03-19 15:05 ` Thomas Gleixner
2018-03-19 15:19 ` David Laight
2018-03-19 15:37 ` Thomas Gleixner
2018-03-19 15:53 ` David Laight
2018-03-19 16:29 ` Linus Torvalds
2018-03-20 8:26 ` Ingo Molnar
2018-03-20 8:38 ` Thomas Gleixner
2018-03-20 9:08 ` Ingo Molnar
2018-03-20 9:41 ` Thomas Gleixner
2018-03-20 9:59 ` David Laight
2018-03-20 10:54 ` Ingo Molnar
2018-03-20 13:30 ` David Laight [this message]
2018-04-03 8:49 ` Pavel Machek
2018-04-03 10:36 ` Ingo Molnar
2018-03-20 14:57 ` Andy Lutomirski
2018-03-20 15:10 ` David Laight
2018-03-21 0:39 ` Andy Lutomirski
2018-03-20 18:01 ` Linus Torvalds
2018-03-21 6:32 ` Ingo Molnar
2018-03-21 15:45 ` Andy Lutomirski
2018-03-22 9:36 ` Ingo Molnar
2018-03-21 7:46 ` Ingo Molnar
2018-03-21 18:15 ` Linus Torvalds
2018-03-22 9:33 ` Ingo Molnar
2018-03-22 17:40 ` Alexei Starovoitov
2018-03-22 17:44 ` Andy Lutomirski
2018-03-22 10:35 ` David Laight
2018-03-22 12:48 ` David Laight
2018-03-22 17:07 ` Linus Torvalds
2018-03-19 15:27 ` Christoph Hellwig
2018-03-20 13:45 ` Rahul Lakkireddy
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