On Thu, Sep 17, 2015 at 07:17:44PM +0300, Imre Deak wrote: > The execlist context object is mapped with a CPU/GPU coherent mapping > everywhere, but on BXT A stepping due to a HW issue the coherency is not > guaranteed. To work around this flush the context object after pinning > it (to flush cache lines left by the context initialization/read-back > from backing storage) and mark it as uncached so later updates during > context switching will be coherent. > > I noticed this problem via a GPU hang, where IPEHR pointed to an invalid > opcode value. I couldn't find this value on the ring but looking at the > contents of the active context object it turned out to be a parameter > dword of a bigger command there. The original command opcode itself > was zeroed out, based on the above I assume due to a CPU writeback of > the corresponding cacheline. When restoring the context the GPU would > jump over the zeroed out opcode and hang when trying to execute the > above parameter dword. > > I could easily reproduce this by running igt/gem_render_copy_redux and > gem_tiled_blits/basic in parallel, but I guess it could be triggered by > anything involving frequent switches between two separate contexts. With > this workaround I couldn't reproduce the problem. > > v2: > - instead of clflushing after updating the tail and PDP values during > context switching, map the corresponding page as uncached to avoid a > race between CPU and GPU, both updating the same cacheline at the same > time (Ville) No. Changing PAT involves a stop_machine() and is severely detrimental to performance (context creation overhead does impact userspace). Mapping it as uncached doesn't remove the race anyway. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
