linux-kernel - Re: [PATCH 2/3] arm64: lib: improve copy performance when size is ge 128 bytes

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Date:   Tue, 23 Mar 2021 15:03:59 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Yang Yingliang <yangyingliang@...wei.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        guohanjun@...wei.com
Subject: Re: [PATCH 2/3] arm64: lib: improve copy performance when size is ge
 128 bytes

On Tue, Mar 23, 2021 at 01:32:18PM +0000, Will Deacon wrote:
> On Tue, Mar 23, 2021 at 12:08:56PM +0000, Robin Murphy wrote:
> > On 2021-03-23 07:34, Yang Yingliang wrote:
> > > When copy over 128 bytes, src/dst is added after
> > > each ldp/stp instruction, it will cost more time.
> > > To improve this, we only add src/dst after load
> > > or store 64 bytes.
> > 
> > This breaks the required behaviour for copy_*_user(), since the fault
> > handler expects the base address to be up-to-date at all times. Say you're
> > copying 128 bytes and fault on the 4th store, it should return 80 bytes not
> > copied; the code below would return 128 bytes not copied, even though 48
> > bytes have actually been written to the destination.
> > 
> > We've had a couple of tries at updating this code (because the whole
> > template is frankly a bit terrible, and a long way from the well-optimised
> > code it was derived from), but getting the fault-handling behaviour right
> > without making the handler itself ludicrously complex has proven tricky. And
> > then it got bumped down the priority list while the uaccess behaviour in
> > general was in flux - now that the dust has largely settled on that I should
> > probably try to find time to pick this up again...
> 
> I think the v5 from Oli was pretty close, but it didn't get any review:
> 
> https://lore.kernel.org/r/20200914151800.2270-1-oli.swede@arm.com

These are still unread in my inbox as I was planning to look at them
again. However, I think we discussed a few options on how to proceed but
no concrete plans:

1. Merge Oli's patches as they are, with some potential complexity
   issues as fixing the user copy accuracy was non-trivial. I think the
   latest version uses a two-stage approach: when taking a fault, it
   falls back to to byte-by-byte with the expectation that it faults
   again and we can then report the correct fault address.

2. Only use Cortex Strings for in-kernel memcpy() while the uaccess
   routines are some simple loops that align the uaccess part only
   (unlike Cortex Strings which usually to align the source).

3. Similar to 2 but with Cortex Strings imported automatically with some
   script to make it easier to keep the routines up to date.

If having non-optimal (but good enough) uaccess routines is acceptable,
I'd go for (2) with a plan to move to (3) at the next Cortex Strings
update.

I also need to look again at option (1) to see how complex it is but
given the time one spends on importing a new Cortex Strings library, I
don't think (1) scales well on the long term. We could, however, go for
(1) now and look at (3) with the next update.

-- 
Catalin