Avi Kivity wrote: > Gregory Haskins wrote: > > >>> BTW, why did you decide to use get_user_pages? >>> Would switch_mm + copy_to_user work as well >>> avoiding page walk if all pages are present? >>> >> >> Well, basic c_t_u() won't work because its likely not "current" if you >> are updating the ring from some other task, but I think you have already >> figured that out based on the switch_mm suggestion. The simple truth is >> I was not familiar with switch_mm at the time I wrote this (nor am I >> now). If this is a superior method that allows you to acquire >> c_t_u(some_other_ctx) like behavior, I see no problem in changing. I >> will look into this, and thanks for the suggestion! >> > > copy_to_user() is significantly faster than get_user_pages() + kmap() > + memcmp() (or their variants). >
Oh, I don't doubt that (in fact, I was pretty sure that was the case based on some of the optimizations I could see in studying the c_t_u() path). I just didn't realize there were other ways to do it if its a non "current" task. ;) I guess the enigma for me right now is what cost does switch_mm have? (Thats not a slam against the suggested approach...I really do not know and am curious). As an aside, note that we seem to be reviewing v2, where v3 is really the last set I pushed. I think this patch is more or less the same across both iterations, but FYI that I would recommend looking at v3 instead. -Greg
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