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@xxxxxxx
he also included tests:
https://lore.kernel.org/r/20200916104636.19172-1-oli.swede@xxxxxxx
It would be great if you or somebody else has time to revive those!