Re: [PATCH v10 3/6] riscv: mm: dma-noncoherent: nonstandard cache operations support

[Guo Ren]

On Mon, Jul 31, 2023 at 4:36 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Sun, Jul 30, 2023, at 17:42, Emil Renner Berthing wrote:
> > On Sun, 30 Jul 2023 at 17:11, Jisheng Zhang <jszhang@xxxxxxxxxx> wrote:
>
> >> > +
> >> >  static inline void arch_dma_cache_wback(phys_addr_t paddr, size_t size)
> >> >  {
> >> >       void *vaddr = phys_to_virt(paddr);
> >> >
> >> > +#ifdef CONFIG_RISCV_NONSTANDARD_CACHE_OPS
> >> > +     if (unlikely(noncoherent_cache_ops.wback)) {
> >>
> >> I'm worried about the performance impact here.
> >> For unified kernel Image reason, RISCV_NONSTANDARD_CACHE_OPS will be
> >> enabled by default, so standard CMO and T-HEAD's CMO platform's
> >> performance will be impacted, because even an unlikely is put
> >> here, the check action still needs to be done.
> >
> > On IRC I asked why not use a static key so the overhead is just a
> > single nop when the standard CMO ops are available, but the consensus
> > seemed to be that the flushing would completely dominate this branch.
> > And on platforms with the standard CMO ops the branch be correctly
> > predicted anyway.
>
> Not just the flushing, but also loading back the invalidated
> cache lines afterwards is just very expensive. I don't think
> you would be able to measure a difference between the static
> key and a correctly predicted branch on any relevant usecase here.
Maybe we should move CMO & THEAD ops to the noncoherent_cache_ops, and
only keep one of them.

I prefer noncoherent_cache_ops, it's more maintance than ALTERNATIVE.

Heiko, How do you think about this?

>
>      Arnd



-- 
Best Regards
 Guo Ren