RISC-V: KVM: Fix guest page fault within HLV* instructions

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: b91f0e4 ("RISC-V: KVM: Factor-out instruction emulation into separate sources")
Signed-off-by: Fangyu Yu <fangyu.yu@linux.alibaba.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20251121133543.46822-1-fangyu.yu@linux.alibaba.com
Signed-off-by: Anup Patel <anup@brainfault.org>

Author: fangyu0809

arch/riscv/kvm/vcpu_insn.c

@@ -298,6 +298,22 @@ static int system_opcode_insn(struct kvm_vcpu *vcpu, struct kvm_run *run,
 	return (rc <= 0) ? rc : 1;
 }
 
+static bool is_load_guest_page_fault(unsigned long 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 a 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.
+	 */
+	return (scause == EXC_LOAD_GUEST_PAGE_FAULT);
+}
+
 /**
  * kvm_riscv_vcpu_virtual_insn -- Handle virtual instruction trap
  *
@@ -323,6 +339,8 @@ int kvm_riscv_vcpu_virtual_insn(struct kvm_vcpu *vcpu, struct kvm_run *run,
 							  ct->sepc,
 							  &utrap);
 			if (utrap.scause) {
+				if (is_load_guest_page_fault(utrap.scause))
+					return 1;
 				utrap.sepc = ct->sepc;
 				kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
 				return 1;
@@ -378,6 +396,8 @@ 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 (is_load_guest_page_fault(utrap.scause))
+				return 1;
 			/* Redirect trap if we failed to read instruction */
 			utrap.sepc = ct->sepc;
 			kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
@@ -504,6 +524,8 @@ 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 (is_load_guest_page_fault(utrap.scause))
+				return 1;
 			/* Redirect trap if we failed to read instruction */
 			utrap.sepc = ct->sepc;
 			kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);