linux-kernel - [PATCH v2 05/12] RISC-V: KVM: Don't flush TLB when PTE is unchanged

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Message-ID: <20250613065743.737102-6-apatel@ventanamicro.com>
Date: Fri, 13 Jun 2025 12:27:36 +0530
From: Anup Patel <apatel@...tanamicro.com>
To: Atish Patra <atish.patra@...ux.dev>
Cc: Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Alexandre Ghiti <alex@...ti.fr>,
	Andrew Jones <ajones@...tanamicro.com>,
	Anup Patel <anup@...infault.org>,
	kvm@...r.kernel.org,
	kvm-riscv@...ts.infradead.org,
	linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Anup Patel <apatel@...tanamicro.com>
Subject: [PATCH v2 05/12] RISC-V: KVM: Don't flush TLB when PTE is unchanged

The gstage_set_pte() and gstage_op_pte() should flush TLB only when
a leaf PTE changes so that unnecessary TLB flushes can be avoided.

Signed-off-by: Anup Patel <apatel@...tanamicro.com>
---
 arch/riscv/kvm/mmu.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
index 1087ea74567b..29f1bd853a66 100644
--- a/arch/riscv/kvm/mmu.c
+++ b/arch/riscv/kvm/mmu.c
@@ -167,9 +167,11 @@ static int gstage_set_pte(struct kvm *kvm, u32 level,
 		ptep = &next_ptep[gstage_pte_index(addr, current_level)];
 	}
 
-	set_pte(ptep, *new_pte);
-	if (gstage_pte_leaf(ptep))
-		gstage_remote_tlb_flush(kvm, current_level, addr);
+	if (pte_val(*ptep) != pte_val(*new_pte)) {
+		set_pte(ptep, *new_pte);
+		if (gstage_pte_leaf(ptep))
+			gstage_remote_tlb_flush(kvm, current_level, addr);
+	}
 
 	return 0;
 }
@@ -229,7 +231,7 @@ static void gstage_op_pte(struct kvm *kvm, gpa_t addr,
 			  pte_t *ptep, u32 ptep_level, enum gstage_op op)
 {
 	int i, ret;
-	pte_t *next_ptep;
+	pte_t old_pte, *next_ptep;
 	u32 next_ptep_level;
 	unsigned long next_page_size, page_size;
 
@@ -258,11 +260,13 @@ static void gstage_op_pte(struct kvm *kvm, gpa_t addr,
 		if (op == GSTAGE_OP_CLEAR)
 			put_page(virt_to_page(next_ptep));
 	} else {
+		old_pte = *ptep;
 		if (op == GSTAGE_OP_CLEAR)
 			set_pte(ptep, __pte(0));
 		else if (op == GSTAGE_OP_WP)
 			set_pte(ptep, __pte(pte_val(ptep_get(ptep)) & ~_PAGE_WRITE));
-		gstage_remote_tlb_flush(kvm, ptep_level, addr);
+		if (pte_val(*ptep) != pte_val(old_pte))
+			gstage_remote_tlb_flush(kvm, ptep_level, addr);
 	}
 }
 
-- 
2.43.0