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

hch
2019-08-20T03:06:45Z

On Mon, Aug 19, 2019 at 05:47:35PM -0700, Atish Patra wrote:
> In RISC-V, tlb flush happens via SBI which is expensive.
> If the target cpumask contains a local hartid, some cost
> can be saved by issuing a local tlb flush as we do that
> in OpenSBI anyways. There is also no need of SBI call if
> cpumask is empty.
>
> Do a local flush first if current cpu is present in cpumask.
> Invoke SBI call only if target cpumask contains any cpus
> other than local cpu.

Btw, you can use up your 70-ish chars per line for commit logs..

> + if (cpumask_test_cpu(cpuid, cmask)) {
> + /* Save trap cost by issuing a local tlb flush here */
> + if ((start == 0 && size == -1) || (size > PAGE_SIZE))
> + local_flush_tlb_all();
> + else if (size == PAGE_SIZE)
> + local_flush_tlb_page(start);
> + }

This looks a little odd to m and assumes we never pass a size smaller
than PAGE_SIZE. Whule that is probably true, why not something like:

if (size < PAGE_SIZE && size != -1)
local_flush_tlb_page(start);
else
local_flush_tlb_all();

?