Re: LDREX and STREX in heterogeneous system？

[richard clark]

On Sat, Nov 12, 2022 at 6:40 AM Catalin Marinas <catalin.marinas at arm.com> wrote:
>
> On Thu, Nov 10, 2022 at 11:27:23AM +0800, richard clark wrote:
> > On Wed, Nov 9, 2022 at 5:26 PM Catalin Marinas <catalin.marinas at arm.com> wrote:
> > >
> > > On Wed, Nov 09, 2022 at 04:34:13PM +0800, richard clark wrote:
> > > > Suppose in a heterogeneous system, there're cortex-M7 and cortex-A72
> > > > sharing the same bus. Does the below code sequence work as (ldr/str)ex
> > > > expected?
> > > >
> > > > r2 point to a uncached shared memory between M7 and A72
> > > >
> > > > M7                                      A72
> > > > ldrex r1, [r2]
> > > >       ------------------------->  strex r0, r1, [r2]
> > >
> > > In general, it won't. The exclusives are supposed to work in the same
> > > inner shareable domain, so it depends on how the SoC has the M7 and A72
> > > wired up. Are they cache coherent with each-other? Is there a global
> > > exclusive monitor? The M7 may also need the MPU regions set up with the
> > > Shareable attribute.
> >
> > Thanks Catalin! AFAIK the M7 and A53 are not in the same Inner
> > shareable domain: if A53 modifies the SRAM with D$ on, the M7 will
> > still get the stale data from the same SRAM location. Except that
> > probably there is not a global exclusive monitor there.
>
> An SoC may allow exclusives on a small range of non-cacheable memory but
> it's not something to infer from the CPUID registers. You'd have to ask
> the hardware people whether they built a global exclusive monitor and
> whether that's shared with the M7.

The hardware people confirmed there's no global exclusive monitor in
the SoC and the M and A are not in the same inner-shareable-domain, so
order to access the shared resources between M and A in an exclusive
way, the SoC provides another method as so-called 'hw gate'...

>
> Otherwise, you might want to look at other algorithms like Lamport's
> bakery.
>
> --
> Catalin