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SubjectRe: [PATCH] RISC-V: Issue a local tlb flush if possible.
Hi Atish,

On Fri, 9 Aug 2019, 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.
>
> Signed-off-by: Atish Patra <atish.patra@wdc.com>

A few brief comments/questions beyond the ones that others have mentioned:

1. At some point, some RISC-V systems may implement this SBI call in
hardware, rather than in software. Then this might wind up becoming a
de-optimization. I don't think there's anything to do about that in code
right now, but it might be worth adding a comment, and thinking about how
that case might be handled in the platform specification group.

2. If this patch masks or reduces the likelihood of hitting the
TLB-related crashes that we're seeing, we probably will want to hold off
on merging this one until we're relatively certain that those other
problems have been fixed.



> ---
> arch/riscv/include/asm/tlbflush.h | 33 +++++++++++++++++++++++++++----
> 1 file changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
> index 687dd19735a7..b32ba4fa5888 100644
> --- a/arch/riscv/include/asm/tlbflush.h
> +++ b/arch/riscv/include/asm/tlbflush.h
> @@ -8,6 +8,7 @@
> #define _ASM_RISCV_TLBFLUSH_H
>
> #include <linux/mm_types.h>
> +#include <linux/sched.h>
> #include <asm/smp.h>
>
> /*
> @@ -46,14 +47,38 @@ static inline void remote_sfence_vma(struct cpumask *cmask, unsigned long start,
> unsigned long size)
> {
> struct cpumask hmask;
> + struct cpumask tmask;
> + int cpuid = smp_processor_id();
>
> cpumask_clear(&hmask);
> - riscv_cpuid_to_hartid_mask(cmask, &hmask);
> - sbi_remote_sfence_vma(hmask.bits, start, size);
> + cpumask_clear(&tmask);
> +
> + if (cmask)
> + cpumask_copy(&tmask, cmask);
> + else
> + cpumask_copy(&tmask, cpu_online_mask);
> +
> + if (cpumask_test_cpu(cpuid, &tmask)) {
> + /* 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);
> + cpumask_clear_cpu(cpuid, &tmask);
> + } else if (cpumask_empty(&tmask)) {
> + /* cpumask is empty. So just do a local flush */
> + local_flush_tlb_all();
> + return;
> + }
> +
> + if (!cpumask_empty(&tmask)) {
> + riscv_cpuid_to_hartid_mask(&tmask, &hmask);
> + sbi_remote_sfence_vma(hmask.bits, start, size);
> + }
> }
>
> -#define flush_tlb_all() sbi_remote_sfence_vma(NULL, 0, -1)
> -#define flush_tlb_page(vma, addr) flush_tlb_range(vma, addr, 0)
> +#define flush_tlb_all() remote_sfence_vma(NULL, 0, -1)
> +#define flush_tlb_page(vma, addr) flush_tlb_range(vma, addr, (addr) + PAGE_SIZE)
> #define flush_tlb_range(vma, start, end) \
> remote_sfence_vma(mm_cpumask((vma)->vm_mm), start, (end) - (start))
> #define flush_tlb_mm(mm) \
> --
> 2.21.0
>
>


- Paul

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