[PATCH v2 4/4] RISC-V: KVM: Add the eager_page_split module parameter

[wang.yechao255 at zte.com.cn]

From: Wang Yechao <wang.yechao255 at zte.com.cn>

Add an eager_page_split module parameter for RISC-V KVM, following
the same approach as on x86. This parameter controls whether eager
page splitting is enabled. The default value is on.

When eager page splitting is enabled, KVM proactively splits large
pages (huge pages) into smaller pages when needed for dirty logging
or other operations. Disabling it can be beneficial for VM workloads
that rarely perform writes, or that only write to a small region of
memory, as it allows huge pages to remain intact for read accesses.

Signed-off-by: Wang Yechao <wang.yechao255 at zte.com.cn>
---
 Documentation/admin-guide/kernel-parameters.txt |  7 +++++--
 arch/riscv/kvm/mmu.c                            | 13 ++++++++++---
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 4d0f545fb3ec..d443b5313b79 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3059,7 +3059,7 @@ Kernel parameters
 			Default is 0 (don't ignore, but inject #GP)

 	kvm.eager_page_split=
-			[KVM,X86] Controls whether or not KVM will try to
+			[KVM,X86,RISCV] Controls whether or not KVM will try to
 			proactively split all huge pages during dirty logging.
 			Eager page splitting reduces interruptions to vCPU
 			execution by eliminating the write-protection faults
@@ -3079,7 +3079,10 @@ Kernel parameters
 			the KVM_CLEAR_DIRTY ioctl, and only for the pages being
 			cleared.

-			Eager page splitting is only supported when kvm.tdp_mmu=Y.
+			On x86, eager page splitting is only supported when
+			kvm.tdp_mmu=Y.
+
+			On RISCV, eager page splitting is supported by default.

 			Default is Y (on).

diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
index d04680687f4a..7a910c5a61fe 100644
--- a/arch/riscv/kvm/mmu.c
+++ b/arch/riscv/kvm/mmu.c
@@ -16,6 +16,9 @@
 #include <asm/kvm_mmu.h>
 #include <asm/kvm_nacl.h>

+bool __read_mostly eager_page_split = true;
+module_param(eager_page_split, bool, 0644);
+
 static void mmu_wp_memory_region(struct kvm *kvm, int slot)
 {
 	struct kvm_memslots *slots = kvm_memslots(kvm);
@@ -150,8 +153,10 @@ void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,

 	kvm_riscv_gstage_wp_range(&gstage, start, end);

-	if (kvm_dirty_log_manual_protect_and_init_set(kvm))
-		mmu_split_huge_pages(&gstage, start, end, true);
+	if (kvm_dirty_log_manual_protect_and_init_set(kvm)) {
+		if (READ_ONCE(eager_page_split))
+			mmu_split_huge_pages(&gstage, start, end, true);
+	}
 }

 void kvm_arch_sync_dirty_log(struct kvm *kvm, struct kvm_memory_slot *memslot)
@@ -216,7 +221,9 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 		if (kvm_dirty_log_manual_protect_and_init_set(kvm))
 			return;
 		mmu_wp_memory_region(kvm, new->id);
-		mmu_split_memory_region(kvm, new->id);
+
+		if (READ_ONCE(eager_page_split))
+			mmu_split_memory_region(kvm, new->id);
 	}
 }

-- 
2.43.5