RE: [PATCH V2 4/4] riscv: mm: Optimize TASK_SIZE definition

[David Laight]

From: Guo Ren
> Sent: 24 December 2023 01:24
...
> > One possibility would be to save the task's max user address
> > in the task structure itself - that would save all the conditionals
> > at a cost of an extra value in the task structure.

> It would still cause memory load operation, although it is $tp->xxx.

All the (mispredicted) branches are likely to cause more of a
problem than a load from the current task structure.

> If we want to gain observability benefits, "just check (ptr | (ptr +
> len)) < 0)" is better.

If you can guarantee a faulting page between user and kernel addresses
and assume (check) that the accesses are 'reasonably sequential'
then you only need to check the base address.
That is likely hard for 32bit but easier for 64bit (except arm64)
because A63 and A62 have to match.
Unless you have some hardware address masking which makes it much
more likely that 'random values' will be valid addresses.
(Someone remind me why that is a good idea unless the high bits
are validated by the hardware.)

	David

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