Messages in this thread | | | Date | Wed, 16 Jun 2021 11:34:46 +0100 | From | "Russell King (Oracle)" <> | Subject | Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE |
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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.
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.
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