[PATCH 2/2] riscv: tlb: avoid tlb flushing if fullmm == 1

[Jisheng Zhang]

The mmu_gather code sets fullmm=1 when tearing down the entire address
space for an mm_struct on exit or execve. So if the underlying platform
supports ASID, the tlb flushing can be avoided because the ASID
allocator will never re-allocate a dirty ASID.

Use the performance of Process creation in unixbench on T-HEAD TH1520
platform is improved by about 4%.

Signed-off-by: Jisheng Zhang <jszhang at kernel.org>
---
 arch/riscv/include/asm/tlb.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/riscv/include/asm/tlb.h b/arch/riscv/include/asm/tlb.h
index 1eb5682b2af6..35f3c214332e 100644
--- a/arch/riscv/include/asm/tlb.h
+++ b/arch/riscv/include/asm/tlb.h
@@ -12,10 +12,19 @@ static void tlb_flush(struct mmu_gather *tlb);
 
 #define tlb_flush tlb_flush
 #include <asm-generic/tlb.h>
+#include <asm/mmu_context.h>
 
 static inline void tlb_flush(struct mmu_gather *tlb)
 {
 #ifdef CONFIG_MMU
+	/*
+	 * If ASID is supported, the ASID allocator will either invalidate the
+	 * ASID or mark it as used. So we can avoid TLB invalidation when
+	 * pulling down a full mm.
+	 */
+	if (static_branch_likely(&use_asid_allocator) && tlb->fullmm)
+		return;
+
 	if (tlb->fullmm || tlb->need_flush_all)
 		flush_tlb_mm(tlb->mm);
 	else
-- 
2.40.0