Re: [PATCH] arm64: Add the arm64.nolse_atomics command line option

[Aiqun(Maria) Yu]

On 7/11/2023 4:22 PM, Will Deacon wrote:
> On Tue, Jul 11, 2023 at 12:02:22PM +0800, Aiqun(Maria) Yu wrote:
> > On 7/10/2023 5:37 PM, Will Deacon wrote:
> > > On Mon, Jul 10, 2023 at 01:59:55PM +0800, Maria Yu wrote:
> > > > In order to be able to disable lse_atomic even if cpu
> > > > support it, most likely because of memory controller
> > > > cannot deal with the lse atomic instructions, use a
> > > > new idreg override to deal with it.
> > >
> > > This should not be a problem for cacheable memory though, right?
> > >
> > > Given that Linux does not issue atomic operations to non-cacheable mappings,
> > > I'm struggling to see why there's a problem here.
> >
> > The lse atomic operation can be issued on non-cacheable mappings as well.
> > Even if it is cached data, with different CPUECTLR_EL1 setting, it can also
> > do far lse atomic operations.
>
> Please can you point me to the place in the kernel sources where this
> happens? The architecture doesn't guarantee that atomics to non-cacheable
> mappings will work, see "B2.2.6 Possible implementation restrictions on
> using atomic instructions". Linux, therefore, doesn't issue atomics
> to non-cacheable memory.

We encounter the issue on third party kernel modules and third party 
apps instead of linux kernel itself.

This is a tradeoff of performance and stability. Per my understanding, 
options can be used to enable the lse_atomic to have the most 
performance cared system, and disable the lse_atomic by stability cared 
most system.
> > > Please can you explain the problem that you are trying to solve?
> >
> > In our current case, it is a 100% reproducible issue that happened for
> > uncached data, the cpu which support LSE atomic, but the system's DDR
> > subsystem is not support this and caused a NOC error and thus synchronous
> > external abort happened.
>
> So? The Arm ARM allows this behaviour and Linux shouldn't run into it.
>
> Will

--
Thx and BRs,
Aiqun(Maria) Yu