Re: [v2 PATCH] RISC-V: Optimize tlb flush path.

Alan Kao
2019-08-21T01:30:41Z

On Tue, Aug 20, 2019 at 08:28:36PM +0000, Atish Patra wrote:
> On Tue, 2019-08-20 at 02:22 -0700, hch@infradead.org wrote:
> > On Tue, Aug 20, 2019 at 08:42:19AM +0000, Atish Patra wrote:
> > > cmask NULL is pretty common case and we would be unnecessarily
> > > executing bunch of instructions everytime while not saving much.
> > > Kernel
> > > still have to make an SBI call and OpenSBI is doing a local flush
> > > anyways.
> > >
> > > Looking at the code again, I think we can just use cpumask_weight
> > > and
> > > do local tlb flush only if local cpu is the only cpu present.
> > >
> > > Otherwise, it will just fall through and call
> > > sbi_remote_sfence_vma().
> >
> > Maybe it is just time to split the different cases at a higher level.
> > The idea to multiple everything onto a single function always seemed
> > odd to me.
> >
> > FYI, here is what I do for the IPI based tlbflush for the native S-
> > mode
> > clint prototype, which seems much easier to understand:
> >
> > http://git.infradead.org/users/hch/riscv.git/commitdiff/ea4067ae61e20fcfcf46a6f6bd1cc25710ce3afe
>
> This does seem a lot cleaner to me. We can reuse some of the code for
> this patch as well. Based on NATIVE_CLINT configuration, it will send
> an IPI or SBI call.

IMHO, this approach should be avoided because CLINT is compatible to but
not mandatory in the privileged spec. In other words, it is possible that
a Linux-capable RISC-V platform does not contain a CLINT component but
rely on some other mechanism to deal with SW/timer interrupts.

>
> I can rebase my patch on top of yours and I can send it together or you
> can include in your series.
>
> Let me know your preference.
>
> --
> Regards,
> Atish

Best,
Alan