[PATCH v3] RISC-V: KVM: Serialize and atomize the IMSIC's TOPEI read-and-clear operation

["BillXiang" <xiangwencheng@lanxincomputing.com>]

The TOPEI read-and-clear must be atomic with respect to
interrupt injection (set_bit in EIP). Otherwise an
interrupt injected between the topei read and the
clear_bit could be silently dropped.

v3:
 - fix undefined eflags

v2:
 - fix patch file curruption

v1: https://lore.kernel.org/kvm-riscv/20260923070332.1376132-1-xiangwencheng@lanxincomputing.com/

Signed-off-by: BillXiang <xiangwencheng@lanxincomputing.com>
---
 arch/riscv/kvm/aia_imsic.c | 42 +++++++++++++++++++++++++-------------
 1 file changed, 28 insertions(+), 14 deletions(-)

diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
index c1af23e79ae0..c440088651ec 100644
--- a/arch/riscv/kvm/aia_imsic.c
+++ b/arch/riscv/kvm/aia_imsic.c
@@ -607,26 +607,25 @@ static void imsic_vsfile_cleanup(struct imsic *imsic)
 		kvm_riscv_aia_free_hgei(old_vsfile_cpu, old_vsfile_hgei);
 }
 
-static void imsic_swfile_extirq_update(struct kvm_vcpu *vcpu)
+static void __imsic_swfile_extirq_update(struct kvm_vcpu *vcpu)
 {
 	struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
 	struct imsic_mrif *mrif = imsic->swfile;
-	unsigned long flags;
-
-	/*
-	 * The critical section is necessary during external interrupt
-	 * updates to avoid the risk of losing interrupts due to potential
-	 * interruptions between reading topei and updating pending status.
-	 */
-
-	raw_spin_lock_irqsave(&imsic->swfile_extirq_lock, flags);
 
 	if (imsic_mrif_atomic_read(mrif, &mrif->eidelivery) &&
 	    imsic_mrif_topei(mrif, imsic->nr_eix, imsic->nr_msis))
 		kvm_riscv_vcpu_set_interrupt(vcpu, IRQ_VS_EXT);
 	else
 		kvm_riscv_vcpu_unset_interrupt(vcpu, IRQ_VS_EXT);
+}
+
+static void imsic_swfile_extirq_update(struct kvm_vcpu *vcpu)
+{
+	struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
+	unsigned long flags;
 
+	raw_spin_lock_irqsave(&imsic->swfile_extirq_lock, flags);
+	__imsic_swfile_extirq_update(vcpu);
 	raw_spin_unlock_irqrestore(&imsic->swfile_extirq_lock, flags);
 }
 
@@ -912,12 +911,15 @@ int kvm_riscv_vcpu_aia_imsic_rmw(struct kvm_vcpu *vcpu, unsigned long isel,
 	struct imsic_mrif_eix *eix;
 	int r, rc = KVM_INSN_CONTINUE_NEXT_SEPC;
 	struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
+	unsigned long flags;
 
 	/* If IMSIC vCPU state not initialized then forward to user space */
 	if (!imsic)
 		return KVM_INSN_EXIT_TO_USER_SPACE;
 
 	if (isel == KVM_RISCV_AIA_IMSIC_TOPEI) {
+		raw_spin_lock_irqsave(&imsic->swfile_extirq_lock, flags);
+
 		/* Read pending and enabled interrupt with highest priority */
 		topei = imsic_mrif_topei(imsic->swfile, imsic->nr_eix,
 					 imsic->nr_msis);
@@ -934,16 +936,20 @@ int kvm_riscv_vcpu_aia_imsic_rmw(struct kvm_vcpu *vcpu, unsigned long isel,
 					  eix->eip);
 			}
 		}
+		if (wr_mask)
+			__imsic_swfile_extirq_update(vcpu);
+
+		raw_spin_unlock_irqrestore(&imsic->swfile_extirq_lock, flags);
 	} else {
 		r = imsic_mrif_rmw(imsic->swfile, imsic->nr_eix, isel,
 				   val, new_val, wr_mask);
 		/* Forward unknown IMSIC register to user-space */
 		if (r)
 			rc = (r == -ENOENT) ? 0 : KVM_INSN_ILLEGAL_TRAP;
-	}
 
-	if (wr_mask)
-		imsic_swfile_extirq_update(vcpu);
+		if (wr_mask)
+			imsic_swfile_extirq_update(vcpu);
+	}
 
 	return rc;
 }
@@ -1050,9 +1056,17 @@ int kvm_riscv_vcpu_aia_imsic_inject(struct kvm_vcpu *vcpu,
 	if (imsic->vsfile_cpu >= 0) {
 		writel(iid, imsic->vsfile_va + IMSIC_MMIO_SETIPNUM_LE);
 	} else {
+		/*
+		 * Serialize the EIP set + external-irq update against the
+		 * TOPEI read-and-clear path so that an injected interrupt
+		 * cannot be silently lost when the guest acknowledges a
+		 * different (or the same) interrupt concurrently.
+		 */
+		raw_spin_lock(&imsic->swfile_extirq_lock);
 		eix = &imsic->swfile->eix[iid / BITS_PER_TYPE(u64)];
 		set_bit(iid & (BITS_PER_TYPE(u64) - 1), eix->eip);
-		imsic_swfile_extirq_update(vcpu);
+		__imsic_swfile_extirq_update(vcpu);
+		raw_spin_unlock(&imsic->swfile_extirq_lock);
 	}
 
 	read_unlock_irqrestore(&imsic->vsfile_lock, flags);
-- 
2.53.0