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

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Message-ID: <aadf2619-4282-f9f3-359c-bb166830f488@arm.com>
Date:   Tue, 23 Mar 2021 14:28:37 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Yang Yingliang <yangyingliang@...wei.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, guohanjun@...wei.com
Subject: Re: [PATCH 2/3] arm64: lib: improve copy performance when size is ge
 128 bytes

On 2021-03-23 13:32, 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
> 
> he also included tests:
> 
> https://lore.kernel.org/r/20200916104636.19172-1-oli.swede@arm.com
> 
> It would be great if you or somebody else has time to revive those!

Yeah, we still have a ticket open for it. Since the uaccess overhaul has 
pretty much killed off any remaining value in the template idea, I might 
have a quick go at spinning a series to just update memcpy() and the 
other library routines to their shiny new versions, then come back and 
work on some dedicated usercopy routines built around LDTR/STTR (and the 
desired fault behaviour) as a follow-up.

(I was also holding out hope for copy_in_user() to disappear if we wait 
long enough, but apparently not yet...)

Robin.