On Tue, Jan 16, 2018 at 10:50 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > On Tue, Jan 16, 2018 at 10:28 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: [..] >> Anything that open-codes copy_from_user() that way is *ALREADY* fucked if >> it cares about the overhead - recent x86 boxen will have slowdown from >> hell on stac()/clac() pairs. Anything like that on a hot path is already >> deep in trouble and needs to be found and fixed. What drivers would those >> be? > > So I took a closer look and the pattern is not copy_from_user it's > more like __get_user + write-to-hardware loops. If the performance is > already expected to be bad for those then perhaps an lfence each loop > iteration won't be much worse. It's still a waste because the lfence > is only needed once after the access_ok. > >> We don't have that many __get_user() users left outside of arch/* >> anymore... Given the concern of having something easy to backport first I think we should start with lfence in __uaccess_begin(). Any deeper changes to the access_ok() + __get_user calling convention can build on top of that baseline.
