On Fri, May 21 2021 at 09:31, Andy Lutomirski wrote: > arch_prctl(SET_XSTATE_INIT_ON_FIRST_USE, TILE_STUFF);? > > As long as this is allowed to fail, I don’t have a huge problem with > it. I'm fine with that. It's still controlled by the OS and can return -EPERM. If allowed then the application would also accept to be insta killed if that #NM allocation fails. Any bug report vs. that will be ignored. > I think several things here are regrettable: > > 1. Legacy XSTATE code might assume that XCR0 is a constant. > > 2. Intel virt really doesn’t like us context switching XCR0, although > we might say that this is Intel’s fault and therefore Intel’s > problem. AMD hardware doesn’t appear to have this issue. > > 3. AMX bring tangled up in XSTATE is unfortunate. The whole XSTATE > mechanism is less than amazing. > > IMO the best we can make of this whole situation is to make XCR0 > dynamic, but the legacy compatibility issues are potentially > problematic. Why? The bit can be enabled and #NM catches the violation of the ABI contract if the application did not request usage. No XCR0 fiddling on context switch required. Thanks, tglx