On Wed, Jun 16, 2021 at 01:10:58PM +0200, Peter Zijlstra wrote: > 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. If it's a problem, then it needs fixing. sys_cacheflush() is used to implement GCC's __builtin___clear_cache(). I'm not sure who added this to gcc. > > 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. Likely only in places where we care about I/D coherency - as the data cache is required to be PIPT on these SMP platforms. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
