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

Christoph Hellwig
2021-05-27T07:09:11Z

On Wed, May 26, 2021 at 05:49:21AM +0000, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.alibaba.com>
>
> 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.

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?

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);
}