Re: [PATCH V4 2/2] riscv: Use use_asid_allocator flush TLB

[Guo Ren]

On Thu, May 27, 2021 at 3:09 PM Christoph Hellwig <hch@xxxxxx> wrote:
>
> On Wed, May 26, 2021 at 05:49:21AM +0000, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > Use static_branch_unlikely(&use_asid_allocator) to keep the origin
> > tlb flush style, so it's no effect on the existing machine. Here
> > are the optimized functions:
> >  - flush_tlb_mm
> >  - flush_tlb_page
> >  - flush_tlb_range
> >
> > All above are based on the below new implement functions:
> >  - __sbi_tlb_flush_range_asid
> >  - local_flush_tlb_range_asid
> >
> > These functions are based on ASID instead of previous non-ASID
> > tlb_flush implementation which invalidates more useful tlb
> > entries.
>
> I still think the commit message is incomplete and rather misleading.
> Here is what I'd come up with from reading the patch:
>
> ---------
> Subject: add ASID-based tlbflushing methods
>
> Implement optimized version of the tlb flushing routines for systems
> using ASIDs.  These are behind the use_asid_allocator static branch to
> not affect existing systems not using ASIDs.
> ---------
>
>
> > +static inline void local_flush_tlb_range_asid(unsigned long start,
> > +                             unsigned long size, unsigned long asid)
> > +{
> > +     unsigned long tmp, end = ALIGN(start + size, PAGE_SIZE);
> > +
> > +     for (tmp = start & PAGE_MASK; tmp < end; tmp += PAGE_SIZE) {
> > +             __asm__ __volatile__ ("sfence.vma %0, %1"
> > +                             :
> > +                             : "r" (tmp), "r" (asid)
> > +                             : "memory");
> > +             tmp += PAGE_SIZE;
> > +     }
>
> This double increments tmp.
Yes, It's a bug for PATCH V4. Thx for point it out.

>
> Also the non-ASID code switches to a global flush once flushing more
> than a single page.  It might be worth documenting the tradeoff in the
> code.
>
> > +static void __sbi_tlb_flush_range_asid(struct cpumask *cmask,
> > +                                    unsigned long start,
> > +                                    unsigned long size,
> > +                                    unsigned long asid)
> > +{
>
> I don't think the calling conventions here are optimal.  I'd pass
> the mm_struct as the first argument, as we can derive both the cpumask
> and asid from it instead of doing that in the callers.
>
> But more importantly I think the static branch check can be moved deeper
> into the code to avoid a lot of duplication.  What do you think of this
> version?
Good idea, but I think "Modifying infrastructure and Adding ASID TLB
flush" should be separated.

I'll try in the next PATCH version.

>
> diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
> index b0659413a080..7030837adc1a 100644
> --- a/arch/riscv/include/asm/mmu_context.h
> +++ b/arch/riscv/include/asm/mmu_context.h
> @@ -33,6 +33,8 @@ static inline int init_new_context(struct task_struct *tsk,
>         return 0;
>  }
>
> +DECLARE_STATIC_KEY_FALSE(use_asid_allocator);
> +
>  #include <asm-generic/mmu_context.h>
>
>  #endif /* _ASM_RISCV_MMU_CONTEXT_H */
> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> index 68aa312fc352..45c1b04b105d 100644
> --- a/arch/riscv/mm/context.c
> +++ b/arch/riscv/mm/context.c
> @@ -18,7 +18,7 @@
>
>  #ifdef CONFIG_MMU
>
> -static DEFINE_STATIC_KEY_FALSE(use_asid_allocator);
> +DEFINE_STATIC_KEY_FALSE(use_asid_allocator);
>
>  static unsigned long asid_bits;
>  static unsigned long num_asids;
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index 720b443c4528..d8afbb1269d5 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -4,6 +4,33 @@
>  #include <linux/smp.h>
>  #include <linux/sched.h>
>  #include <asm/sbi.h>
> +#include <asm/mmu_context.h>
> +
> +static inline void local_flush_tlb_all_asid(unsigned long asid)
> +{
> +       __asm__ __volatile__ ("sfence.vma x0, %0"
> +                       :
> +                       : "r" (asid)
> +                       : "memory");
> +}
> +
> +static inline void local_flush_tlb_page_asid(unsigned long addr,
> +               unsigned long asid)
> +{
> +       __asm__ __volatile__ ("sfence.vma %0, %1"
> +                       :
> +                       : "r" (addr), "r" (asid)
> +                       : "memory");
> +}
> +
> +static inline void local_flush_tlb_range_asid(unsigned long start,
> +                               unsigned long size, unsigned long asid)
> +{
> +       unsigned long addr, end = ALIGN(start + size, PAGE_SIZE);
> +
> +       for (addr = start & PAGE_MASK; addr < end; addr += PAGE_SIZE)
> +               local_flush_tlb_page_asid(addr, asid);
> +}
>
>  void flush_tlb_all(void)
>  {
> @@ -12,28 +39,43 @@ void flush_tlb_all(void)
>
>  /*
>   * This function must not be called with cmask being null.
> - * Kernel may panic if cmask is NULL.
>   */
> -static void __sbi_tlb_flush_range(struct cpumask *cmask, unsigned long start,
> +static void __sbi_tlb_flush_range(struct mm_struct *mm, unsigned long start,
>                                   unsigned long size)
>  {
> +       struct cpumask *cmask = mm_cpumask(mm);
>         struct cpumask hmask;
>         unsigned int cpuid;
> +       bool broadcast;
>
>         if (cpumask_empty(cmask))
>                 return;
>
>         cpuid = get_cpu();
> +       /* check if the tlbflush needs to be sent to other CPUs */
> +       broadcast = cpumask_any_but(cmask, cpuid) < nr_cpu_ids;
> +       if (static_branch_unlikely(&use_asid_allocator)) {
> +               unsigned long asid = atomic_long_read(&mm->context.id);
>
> -       if (cpumask_any_but(cmask, cpuid) >= nr_cpu_ids) {
> -               /* local cpu is the only cpu present in cpumask */
> -               if (size <= PAGE_SIZE)
> +               if (broadcast) {
> +                       riscv_cpuid_to_hartid_mask(cmask, &hmask);
> +                       sbi_remote_sfence_vma_asid(cpumask_bits(&hmask),
> +                                                  start, size, asid);
> +               } else if (size != -1) {
> +                       local_flush_tlb_range_asid(start, size, asid);
> +               } else {
> +                       local_flush_tlb_all_asid(asid);
> +               }
> +       } else {
> +               if (broadcast) {
> +                       riscv_cpuid_to_hartid_mask(cmask, &hmask);
> +                       sbi_remote_sfence_vma(cpumask_bits(&hmask),
> +                                             start, size);
> +               } else if (size <= PAGE_SIZE) {
>                         local_flush_tlb_page(start);
> -               else
> +               } else {
>                         local_flush_tlb_all();
> -       } else {
> -               riscv_cpuid_to_hartid_mask(cmask, &hmask);
> -               sbi_remote_sfence_vma(cpumask_bits(&hmask), start, size);
> +               }
>         }
>
>         put_cpu();
> @@ -41,16 +83,16 @@ static void __sbi_tlb_flush_range(struct cpumask *cmask, unsigned long start,
>
>  void flush_tlb_mm(struct mm_struct *mm)
>  {
> -       __sbi_tlb_flush_range(mm_cpumask(mm), 0, -1);
> +       __sbi_tlb_flush_range(mm, 0, -1);
>  }
>
>  void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
>  {
> -       __sbi_tlb_flush_range(mm_cpumask(vma->vm_mm), addr, PAGE_SIZE);
> +       __sbi_tlb_flush_range(vma->vm_mm, addr, PAGE_SIZE);
>  }
>
>  void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
>                      unsigned long end)
>  {
> -       __sbi_tlb_flush_range(mm_cpumask(vma->vm_mm), start, end - start);
> +       __sbi_tlb_flush_range(vma->vm_mm, start, end - start);
>  }



-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/