On Thu, Nov 19, 2020 at 03:19:14PM +0100, Peter Zijlstra wrote: > On Thu, Nov 19, 2020 at 01:25:11PM +0000, Chris Wilson wrote: > > Quoting Peter Zijlstra (2020-11-19 13:02:44) > > > > > > Chris, I suspect this is due to i915 calling stop machine with all sorts > > > of locks held. Is there anything to be done about this? stop_machine() > > > is really nasty to begin with. > > > > > > What problem is it typing to solve? > > > > If there is any concurrent access through a PCI bar (that is exported to > > userspace via mmap) as the GTT is updated, results in undefined HW > > behaviour (where that is not limited to users writing to other system > > pages). > > > > stop_machine() is the most foolproof method we know that works. > > Sorry, I don't understand. It tries to do what? And why does it need to > do that holding locks. > > Really, this is very bad form.
Having poked around at the code; do I get it correct that this is using stop-machine to set IOMMU page-table entries, because the hardware cannot deal with two CPUs writing to the same device page-tables; which would be possible because that memory is exposed through PCI bars? Can't you simply exclude that memory from being visible through the PCI bar crud? Having to use stop-machine seems tragic, doubly so because nobody should actually be having that memory mapped in the first place.
