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

[Christoph Hellwig <hch@lst.de>]

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);
 }