Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: > On Sun, Oct 13, 2019 at 12:59 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: >> Re plotting: how strongly would you object against passing the range to >> user_access_end()? Powerpc folks have a very close analogue of stac/clac, >> currently buried inside their __get_user()/__put_user()/etc. - the same >> places where x86 does, including futex.h and friends. >> >> And there it's even costlier than on x86. It would obviously be nice >> to lift it at least out of unsafe_get_user()/unsafe_put_user() and >> move into user_access_begin()/user_access_end(); unfortunately, in >> one subarchitecture they really want it the range on the user_access_end() >> side as well. > > Hmm. I'm ok with that. > > Do they want the actual range, or would it prefer some kind of opaque > cookie that user_access_begin() returns (where 0 would mean "failure" > of course)? The code does want the actual range, or at least the range rounded to a segment boundary (256MB). But it can get that already from a value it stashes in current->thread, it was just more natural to pass the addr/size with the way the code is currently structured. It seems to generate slightly better code to pass addr/size vs loading it from current->thread, but it's probably in the noise vs everything else that's going on. So a cookie would work fine, we could return the encoded addr/size in the cookie and that might generate better code than loading it back from current->thread. Equally we could just use the value in current->thread and not have any cookie at all. cheers
