Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE

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:

quoted

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.

-- 
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