[PATCH v2 1/2] KVM: RISC-V: Clear former VCPU cache on virtualization disable

[Yong-Xuan Wang <yongxuan.wang@sifive.com>]

When a CPU is taken offline or enters deep idle states, hypervisor CSR
state is lost. The kvm_former_vcpu fast-path optimization caches the
last VCPU that ran on each CPU to avoid expensive CSR restoration when
the same VCPU is re-scheduled on the same CPU. However, if this cache
is not cleared when CSR state is lost, the next VCPU entry will
incorrectly skip CSR restoration, leading to corrupt hypervisor state.

Add kvm_riscv_clear_former_vcpu() to invalidate the per-CPU cache and
call it from kvm_arch_disable_virtualization_cpu() to ensure proper CSR
restoration after CPU offline or system suspend events.

Fixes: 1323a5cfe52c ("KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core")

Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
---
 arch/riscv/include/asm/kvm_host.h |  2 ++
 arch/riscv/kvm/main.c             |  2 ++
 arch/riscv/kvm/vcpu.c             | 11 +++++++++++
 3 files changed, 15 insertions(+)

diff --git a/arch/riscv/include/asm/kvm_host.h b/arch/riscv/include/asm/kvm_host.h
index 60017ceec9d2..808b4ee1bd10 100644
--- a/arch/riscv/include/asm/kvm_host.h
+++ b/arch/riscv/include/asm/kvm_host.h
@@ -319,6 +319,8 @@ bool kvm_riscv_vcpu_stopped(struct kvm_vcpu *vcpu);
 
 void kvm_riscv_vcpu_record_steal_time(struct kvm_vcpu *vcpu);
 
+void kvm_riscv_clear_former_vcpu(void);
+
 /* Flags representing implementation specific details */
 DECLARE_STATIC_KEY_FALSE(kvm_riscv_vsstage_tlb_no_gpa);
 
diff --git a/arch/riscv/kvm/main.c b/arch/riscv/kvm/main.c
index 0924c75100a2..350e4f097d6e 100644
--- a/arch/riscv/kvm/main.c
+++ b/arch/riscv/kvm/main.c
@@ -69,6 +69,8 @@ void kvm_arch_disable_virtualization_cpu(void)
 	csr_write(CSR_HEDELEG, 0);
 	csr_write(CSR_HIDELEG, 0);
 
+	kvm_riscv_clear_former_vcpu();
+
 	kvm_riscv_nacl_disable();
 }
 
diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
index cf6e231e76e2..e48338f907f4 100644
--- a/arch/riscv/kvm/vcpu.c
+++ b/arch/riscv/kvm/vcpu.c
@@ -26,6 +26,17 @@
 
 static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_former_vcpu);
 
+void kvm_riscv_clear_former_vcpu(void)
+{
+	/*
+	 * Clear the per-CPU former VCPU pointer because hypervisor CSR state
+	 * will be lost. This ensures that the next VCPU entry will properly
+	 * restore all CSRs instead of incorrectly skipping CSR restoration
+	 * via the fast-path optimization.
+	 */
+	__this_cpu_write(kvm_former_vcpu, NULL);
+}
+
 const struct kvm_stats_desc kvm_vcpu_stats_desc[] = {
 	KVM_GENERIC_VCPU_STATS(),
 	STATS_DESC_COUNTER(VCPU, ecall_exit_stat),

-- 
2.43.7