linux-kernel - Re: [PATCH] RISC-V: Issue a local tlb flush if possible.

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Date:   Sat, 10 Aug 2019 09:21:45 +0000
From:   Atish Patra <Atish.Patra@....com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Albert Ou <aou@...s.berkeley.edu>,
        Alexios Zavras <alexios.zavras@...el.com>,
        Allison Randal <allison@...utok.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Anup Patel <Anup.Patel@....com>
Subject: Re: [PATCH] RISC-V: Issue a local tlb flush if possible.



On 8/9/19, 6:43 PM, "Atish Patra" <Atish.Patra@....com> 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@....com>
    ---
     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)

I did not see Paul's patch to fix this. I will remove this in v2 and rebase on top of Paul's patch.

Regards,
Atish
     #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