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

Andy Lutomirski
2021-06-17T13:44:13Z

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.