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

[Aiqun(Maria) Yu]

On 7/11/2023 6:25 PM, Will Deacon wrote:
> On Tue, Jul 11, 2023 at 06:15:49PM +0800, Aiqun(Maria) Yu wrote:
> > 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.
>
> Great, so there's nothing to do in the kernel then!
>
> The third party code needs to be modified not to use atomic instructions
> with non-cacheable mappings. No need to involve us with that.

> > 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.
>
> Where do livelock and starvation fit in with "stability"? Disabling LSE
> atomics for things like qspinlock and the scheduler just because of some
> badly written third-party code isn't much of a tradeoff.
We also have requirement to have cpus/system fully support lse atomic 
and cpus/system not fully support lse atomic with a generic kernel image.
Same kernel module wanted to be used by lse atomic fully support cpu and 
not fully support cpu/system as well.

That's why we want to have a runtime option here.

> Will

--
Thx and BRs,
Aiqun(Maria) Yu