On Thu, Apr 02, 2020 at 12:30:33PM +0100, Catalin Marinas wrote: > On Mon, Mar 30, 2020 at 04:32:31PM +0200, Ard Biesheuvel wrote: > > Could we make this a runtime thing? E.g., remap the entire linear > > region down to pages under stop_machine() the first time we probe a > > device that uses non-coherent DMA? > > That could be pretty expensive at run-time. With the ARMv8.4-TTRem > feature, I wonder whether we could do this lazily when allocating > non-coherent DMA buffers. It's worth noting that ARMv8.4-TTRem is optional and the "level 1" and "level 2" behaviours still allows non-fatal faults to be taken while nT is set (or until you perform the TLB invalidation). We can only safely use it where we could use a full BBM sequence today. Effectively TTRem is an optimized, where the CPU *might* be able to use entries during the break stage, but is not guaranteed to do so. Thanksm Mark.
