On Thu, Jun 17, 2021, at 4:33 AM, Mark Rutland wrote: > On Thu, Jun 17, 2021 at 12:23:05PM +0100, Russell King (Oracle) wrote: > > On Thu, Jun 17, 2021 at 11:40:46AM +0100, Mark Rutland wrote: > > > On Tue, Jun 15, 2021 at 08:21:12PM -0700, Andy Lutomirski wrote: > > > > On arm32, the only way to safely flush icache from usermode is to call > > > > cacheflush(2). This also handles any required pipeline flushes, so > > > > membarrier's SYNC_CORE feature is useless on arm. Remove it. > > > > > > Unfortunately, it's a bit more complicated than that, and these days > > > SYNC_CORE is equally necessary on arm as on arm64. This is something > > > that changed in the architecture over time, but since ARMv7 we generally > > > need both the cache maintenance *and* a context synchronization event > > > (the latter must occur on the CPU which will execute the instructions). > > > > > > If you look at the latest ARMv7-AR manual (ARM DDI 406C.d), section > > > A3.5.4 "Concurrent modification and execution of instructions" covers > > > this. That manual can be found at: > > > > > > https://developer.arm.com/documentation/ddi0406/latest/ > > > > Looking at that, sys_cacheflush() meets this. The manual details a > > series of cache maintenance calls in "step 1" that the modifying thread > > must issue - this is exactly what sys_cacheflush() does. The same is > > true for ARMv6, except the "ISB" terminology is replaced by a > > "PrefetchFlush" terminology. (I checked DDI0100I). > > > > "step 2" requires an ISB on the "other CPU" prior to executing that > > code. As I understand it, in ARMv7, userspace can issue an ISB itself. > > > > For ARMv6K, it doesn't have ISB, but instead has a CP15 instruction > > for this that isn't availble to userspace. This is where we come to > > the situation about ARM 11MPCore, and whether we continue to support > > it or not. > > > > So, I think we're completely fine with ARMv7 under 32-bit ARM kernels > > as userspace has everything that's required. ARMv6K is a different > > matter as we've already identified for several reasons. > > Sure, and I agree we should not change cacheflush(). > > The point of membarrier(SYNC_CORE) is that you can move the cost of that > ISB out of the fast-path in the executing thread(s) and into the > slow-path on the thread which generated the code. > > So e.g. rather than an executing thread always having to do: > > LDR <reg>, [<funcptr>] > ISB // in case funcptr was just updated > BLR <reg> > > ... you have the thread generating the code use membarrier(SYNC_CORE) > prior to plublishing the funcptr, and the fast-path on all the executing > threads can be: > > LDR <reg> [<funcptr>] > BLR <reg> > > ... and thus I think we still want membarrier(SYNC_CORE) so that people > can do this, even if there are other means to achieve the same > functionality. I had the impression that sys_cacheflush() did that. Am I wrong? In any event, I’m even more convinced that no new SYNC_CORE arches should be added. We need a new API that just does the right thing.
