[PATCH v2] RISC-V: KVM: Fix guest page fault within HLV* instructions

[fangyu.yu@linux.alibaba.com]

From: Fangyu Yu <fangyu.yu@linux.alibaba.com>

When executing HLV* instructions at the HS mode, a guest page fault
may occur when a g-stage page table migration between triggering the
virtual instruction exception and executing the HLV* instruction.

This may be a corner case, and one simpler way to handle this is to
re-execute the instruction where the virtual  instruction exception
occurred, and the guest page fault will be automatically handled.

Fixes: b91f0e4cb8a3 ("RISC-V: KVM: Factor-out instruction emulation into separate sources")
Signed-off-by: Fangyu Yu <fangyu.yu@linux.alibaba.com>

---
Changes in v2:
- Remove unnecessary modifications and add comments(suggested by Anup)
- Update Fixes tag
- Link to v1: https://lore.kernel.org/linux-riscv/20250912134332.22053-1-fangyu.yu@linux.alibaba.com/
---
 arch/riscv/kvm/vcpu_insn.c | 39 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/arch/riscv/kvm/vcpu_insn.c b/arch/riscv/kvm/vcpu_insn.c
index de1f96ea6225..a8d796ef2822 100644
--- a/arch/riscv/kvm/vcpu_insn.c
+++ b/arch/riscv/kvm/vcpu_insn.c
@@ -323,6 +323,19 @@ int kvm_riscv_vcpu_virtual_insn(struct kvm_vcpu *vcpu, struct kvm_run *run,
 							  ct->sepc,
 							  &utrap);
 			if (utrap.scause) {
+				/**
+				 * If a g-stage page fault occurs, the direct approach
+				 * is to let the g-stage page fault handler handle it
+				 * naturally, however, calling the g-stage page fault
+				 * handler here seems rather strange.
+				 * Considering this is an corner case, we can directly
+				 * return to the guest and re-execute the same PC, this
+				 * will trigger a g-stage page fault again and then the
+				 * regular g-stage page fault handler will populate
+				 * g-stage page table.
+				 */
+				if (utrap.scause == EXC_LOAD_GUEST_PAGE_FAULT)
+					return 1;
 				utrap.sepc = ct->sepc;
 				kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
 				return 1;
@@ -378,6 +391,19 @@ int kvm_riscv_vcpu_mmio_load(struct kvm_vcpu *vcpu, struct kvm_run *run,
 		insn = kvm_riscv_vcpu_unpriv_read(vcpu, true, ct->sepc,
 						  &utrap);
 		if (utrap.scause) {
+			/**
+			 * If a g-stage page fault occurs, the direct approach
+			 * is to let the g-stage page fault handler handle it
+			 * naturally, however, calling the g-stage page fault
+			 * handler here seems rather strange.
+			 * Considering this is an corner case, we can directly
+			 * return to the guest and re-execute the same PC, this
+			 * will trigger a g-stage page fault again and then the
+			 * regular g-stage page fault handler will populate
+			 * g-stage page table.
+			 */
+			if (utrap.scause == EXC_LOAD_GUEST_PAGE_FAULT)
+				return 1;
 			/* Redirect trap if we failed to read instruction */
 			utrap.sepc = ct->sepc;
 			kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
@@ -504,6 +530,19 @@ int kvm_riscv_vcpu_mmio_store(struct kvm_vcpu *vcpu, struct kvm_run *run,
 		insn = kvm_riscv_vcpu_unpriv_read(vcpu, true, ct->sepc,
 						  &utrap);
 		if (utrap.scause) {
+			/**
+			 * If a g-stage page fault occurs, the direct approach
+			 * is to let the g-stage page fault handler handle it
+			 * naturally, however, calling the g-stage page fault
+			 * handler here seems rather strange.
+			 * Considering this is an corner case, we can directly
+			 * return to the guest and re-execute the same PC, this
+			 * will trigger a g-stage page fault again and then the
+			 * regular g-stage page fault handler will populate
+			 * g-stage page table.
+			 */
+			if (utrap.scause == EXC_LOAD_GUEST_PAGE_FAULT)
+				return 1;
 			/* Redirect trap if we failed to read instruction */
 			utrap.sepc = ct->sepc;
 			kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
-- 
2.50.1