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Re: Unifying /dev/{mem,kmem,zero,null} implementations

Joerg Sonnenberger wrote:
On Sun, Feb 24, 2008 at 12:40:51AM +0000, Chris Gilbert wrote:
The documentation says every pmap_kremove needs a pmap_update, relying on something later on doing a pmap_update is risky.

Not here. The address is used strictly for one purpose and a CPU can
only use it when holding the mutex. So the pmap_update after
pmap_kremove shouldn't be needed.

In theory pmap_kremove could be optimised to not change anything in hardware, and defer the work to be done till the pmap_update, while I don't believe any platform has gone to that level yet, we shouldn't exclude the possibility by not following the documented API.

I don't read the documentation that way. pmap_update will happen at a
later point anyway and that should be good enough.

pmap(9) man page for pmap_update says:

This routine must be used after calls to pmap_enter(), pmap_remove(), pmap_protect(), pmap_kenter_pa(), and pmap_kremove() in order to ensure correct operation of the virtual memory system.

Perhaps I'm being pedantic but that says pmap_update is needed. If we want to make it optional we should probably state it explicitly, or perhaps make platforms flush kernel pmap updates out before going back to userland.

I guess my concern is that people like to copy existing code, and it might get missed when it is actually important.


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