On Wed, Jun 16, 2021 at 11:34:46AM +0100, Russell King (Oracle) wrote: > On Wed, Jun 16, 2021 at 12:20:06PM +0200, Peter Zijlstra wrote: > > On Wed, Jun 16, 2021 at 12:16:27PM +0200, Peter Zijlstra 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. > > > > > > So SYNC_CORE is there to help an architecture that needs to do something > > > per CPU. If I$ invalidation is broadcast and I$ invalidation also > > > triggers the flush of any uarch caches derived from it (if there are > > > any). > > > > Incomplete sentence there: + then we don't need SYNC_CORE. > > > > > Now arm_syscall() NR(cacheflush) seems to do flush_icache_user_range(), > > > which, if I read things right, end up in arch/arm/mm/*.S, but that > > > doesn't consider cache_ops_need_broadcast(). > > > > > > Will suggests that perhaps ARM 11MPCore might need this due to their I$ > > > flush maybe not being broadcast > > If it leaves other cores with incoherent I cache, then that's already > a problem for SMP cores, since there could be no guarantee that the > modifications made by one core will be visible to some other core that > ends up running that code - and there is little option for userspace to > work around that except by pinning the thread making the modifications > and subsequently executing the code to a core. That's where SYNC_CORE can help. Or you make sys_cacheflush() do a system wide IPI. > The same is also true of flush_icache_range() - which is used when > loading a kernel module. In the case Will is referring to, these alias > to the same code. Yes, cache_ops_need_broadcast() seems to be missing in more places.
