Messages in this thread | | | Date | Thu, 1 Jul 2004 02:50:47 +0100 | From | Jamie Lokier <> | Subject | Re: A question about PROT_NONE on ARM and ARM26 |
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Nicolas Pitre wrote: > > cmp r0, #(TASK_SIZE - (1<<24)) > > > > I.e. just compare against the largest constant that can be > > represented. For accesses to the last part of userspace, it's a > > penalty of 4 instructions -- but it might work out to be a net gain. > > Maybe not. The user stack is located at the top so any user buffer > allocated on the stack would be penalized.
I agree. I don't know if it would work out to be a net gain on average or a net loss.
It saves a couple of instructions, but when it fails the cost is only a few instructions anyway.
Probably for get_user & put_user, the common case _is_ to be on the user's stack, so Russell's code would be better.
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