Messages in this thread | | | Date | Tue, 13 Aug 2019 11:25:15 -0700 (PDT) | From | Paul Walmsley <> | Subject | Re: [PATCH] RISC-V: Issue a local tlb flush if possible. |
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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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