[PATCH v2 1/1] RISC-V: KVM: Serialize virtual interrupt pending state updates
Xie Bo
xb at ultrarisc.com
Tue Jul 7 20:35:36 PDT 2026
RISC-V KVM tracks guest interrupt state with two bitmaps:
- irqs_pending: interrupts that should be visible to the guest
- irqs_pending_mask: interrupts whose pending state changed
The current code updates those bitmaps with independent atomic bitops
and assumes a multiple-producer, single-consumer protocol. That model
does not actually hold.
kvm_riscv_vcpu_sync_interrupts() is not a pure consumer. When the guest
changes guest-visible HVIP state, sync_interrupts() writes both
irqs_pending and irqs_pending_mask to reflect the new guest state back
into KVM state. As a result, irqs_pending and irqs_pending_mask form a
single logical state transition, but they are not updated atomically as
a pair.
This allows a race where a newly injected interrupt is lost. For
example:
CPU0 CPU1
---- ----
kvm_riscv_vcpu_set_interrupt(VS_SOFT)
set_bit(VS_SOFT, irqs_pending)
kvm_riscv_vcpu_sync_interrupts()
sees guest-cleared HVIP.VSSIP
sets irqs_pending_mask
clears irqs_pending
set_bit(VS_SOFT, irqs_pending_mask)
kvm_vcpu_kick()
After that interleaving, a later flush can update HVIP without VSSIP
even though a new virtual interrupt was injected. In practice, the
guest can remain blocked in WFI with work pending.
The same pending/mask protocol is shared by VS soft interrupts, PMU
overflow delivery, and AIA high interrupt synchronization, so the race
is not limited to one interrupt source.
Fix this by serializing all updates to irqs_pending and
irqs_pending_mask with a per-vCPU raw spinlock. This keeps the pending
bit and the dirty mask as one state transition across:
- set/unset interrupt
- guest HVIP sync
- interrupt flush to guest CSR state
- vCPU reset
- AIA CSR writes that clear dirty state
This intentionally replaces the existing lockless protocol instead of
trying to repair it with additional barriers. The problem is not memory
ordering on a single field; it is that two separate bitmaps encode one
shared state machine while both producers and sync paths can modify
them. A per-vCPU raw spinlock keeps the fix small, local, and suitable
for backporting.
Observed symptom:
- guest occasionally remains blocked in WFI after host-side virtual
interrupt injection
Testing:
- reproduced on a RISC-V KVM setup running lkvm stress workloads
- observed guest stalls with WFI diagnostics while host-side kick and
interrupt injection had already happened
- stress run no longer reproduced the lost-VSSIP hang after applying
this change
Fixes: cce69aff689e ("RISC-V: KVM: Implement VCPU interrupts and requests handling")
Cc: stable at vger.kernel.org
Signed-off-by: Xie Bo <xb at ultrarisc.com>
---
arch/riscv/include/asm/kvm_host.h | 10 +++++-----
arch/riscv/kvm/aia.c | 28 +++++++++++++++++++++-------
arch/riscv/kvm/vcpu.c | 27 ++++++++++++++++++++++-----
arch/riscv/kvm/vcpu_onereg.c | 13 +++++++++----
4 files changed, 57 insertions(+), 21 deletions(-)
diff --git a/arch/riscv/include/asm/kvm_host.h b/arch/riscv/include/asm/kvm_host.h
index 75b0a951c..97e42645c 100644
--- a/arch/riscv/include/asm/kvm_host.h
+++ b/arch/riscv/include/asm/kvm_host.h
@@ -207,13 +207,13 @@ struct kvm_vcpu_arch {
/*
* VCPU interrupts
*
- * We have a lockless approach for tracking pending VCPU interrupts
- * implemented using atomic bitops. The irqs_pending bitmap represent
- * pending interrupts whereas irqs_pending_mask represent bits changed
- * in irqs_pending. Our approach is modeled around multiple producer
- * and single consumer problem where the consumer is the VCPU itself.
+ * The irqs_pending bitmap represents pending interrupts whereas
+ * irqs_pending_mask represents bits changed in irqs_pending. Updates
+ * to these bitmaps are serialized so vcpu interrupt sync/flush cannot
+ * drop a newly injected interrupt while syncing guest-visible HVIP.
*/
#define KVM_RISCV_VCPU_NR_IRQS 64
+ raw_spinlock_t irqs_pending_lock;
DECLARE_BITMAP(irqs_pending, KVM_RISCV_VCPU_NR_IRQS);
DECLARE_BITMAP(irqs_pending_mask, KVM_RISCV_VCPU_NR_IRQS);
diff --git a/arch/riscv/kvm/aia.c b/arch/riscv/kvm/aia.c
index 5ec503288..821d2cb6d 100644
--- a/arch/riscv/kvm/aia.c
+++ b/arch/riscv/kvm/aia.c
@@ -50,17 +50,21 @@ void kvm_riscv_vcpu_aia_flush_interrupts(struct kvm_vcpu *vcpu)
{
struct kvm_vcpu_aia_csr *csr = &vcpu->arch.aia_context.guest_csr;
unsigned long mask, val;
+ unsigned long flags;
if (!kvm_riscv_aia_available())
return;
- if (READ_ONCE(vcpu->arch.irqs_pending_mask[1])) {
- mask = xchg_acquire(&vcpu->arch.irqs_pending_mask[1], 0);
- val = READ_ONCE(vcpu->arch.irqs_pending[1]) & mask;
+ raw_spin_lock_irqsave(&vcpu->arch.irqs_pending_lock, flags);
+ mask = vcpu->arch.irqs_pending_mask[1];
+ if (mask) {
+ vcpu->arch.irqs_pending_mask[1] = 0;
+ val = vcpu->arch.irqs_pending[1] & mask;
csr->hviph &= ~mask;
csr->hviph |= val;
}
+ raw_spin_unlock_irqrestore(&vcpu->arch.irqs_pending_lock, flags);
}
void kvm_riscv_vcpu_aia_sync_interrupts(struct kvm_vcpu *vcpu)
@@ -205,6 +209,9 @@ int kvm_riscv_vcpu_aia_set_csr(struct kvm_vcpu *vcpu,
{
struct kvm_vcpu_aia_csr *csr = &vcpu->arch.aia_context.guest_csr;
unsigned long regs_max = sizeof(struct kvm_riscv_aia_csr) / sizeof(unsigned long);
+#ifdef CONFIG_32BIT
+ unsigned long flags;
+#endif
if (!riscv_isa_extension_available(vcpu->arch.isa, SSAIA))
return -ENOENT;
@@ -214,11 +221,18 @@ int kvm_riscv_vcpu_aia_set_csr(struct kvm_vcpu *vcpu,
reg_num = array_index_nospec(reg_num, regs_max);
if (kvm_riscv_aia_available()) {
- ((unsigned long *)csr)[reg_num] = val;
-
#ifdef CONFIG_32BIT
- if (reg_num == KVM_REG_RISCV_CSR_AIA_REG(siph))
- WRITE_ONCE(vcpu->arch.irqs_pending_mask[1], 0);
+ if (reg_num == KVM_REG_RISCV_CSR_AIA_REG(siph)) {
+ raw_spin_lock_irqsave(&vcpu->arch.irqs_pending_lock, flags);
+ ((unsigned long *)csr)[reg_num] = val;
+ vcpu->arch.irqs_pending_mask[1] = 0;
+ raw_spin_unlock_irqrestore(&vcpu->arch.irqs_pending_lock,
+ flags);
+ } else {
+ ((unsigned long *)csr)[reg_num] = val;
+ }
+#else
+ ((unsigned long *)csr)[reg_num] = val;
#endif
}
diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
index a73690eda..b04730ccd 100644
--- a/arch/riscv/kvm/vcpu.c
+++ b/arch/riscv/kvm/vcpu.c
@@ -80,6 +80,7 @@ static void kvm_riscv_vcpu_context_reset(struct kvm_vcpu *vcpu,
static void kvm_riscv_reset_vcpu(struct kvm_vcpu *vcpu, bool kvm_sbi_reset)
{
+ unsigned long flags;
bool loaded;
/**
@@ -104,8 +105,10 @@ static void kvm_riscv_reset_vcpu(struct kvm_vcpu *vcpu, bool kvm_sbi_reset)
kvm_riscv_vcpu_aia_reset(vcpu);
+ raw_spin_lock_irqsave(&vcpu->arch.irqs_pending_lock, flags);
bitmap_zero(vcpu->arch.irqs_pending, KVM_RISCV_VCPU_NR_IRQS);
bitmap_zero(vcpu->arch.irqs_pending_mask, KVM_RISCV_VCPU_NR_IRQS);
+ raw_spin_unlock_irqrestore(&vcpu->arch.irqs_pending_lock, flags);
kvm_riscv_vcpu_pmu_reset(vcpu);
@@ -151,6 +154,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
/* Setup VCPU hfence queue */
spin_lock_init(&vcpu->arch.hfence_lock);
+ raw_spin_lock_init(&vcpu->arch.irqs_pending_lock);
spin_lock_init(&vcpu->arch.reset_state.lock);
@@ -352,14 +356,18 @@ void kvm_riscv_vcpu_flush_interrupts(struct kvm_vcpu *vcpu)
{
struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
unsigned long mask, val;
+ unsigned long flags;
- if (READ_ONCE(vcpu->arch.irqs_pending_mask[0])) {
- mask = xchg_acquire(&vcpu->arch.irqs_pending_mask[0], 0);
- val = READ_ONCE(vcpu->arch.irqs_pending[0]) & mask;
+ raw_spin_lock_irqsave(&vcpu->arch.irqs_pending_lock, flags);
+ mask = vcpu->arch.irqs_pending_mask[0];
+ if (mask) {
+ vcpu->arch.irqs_pending_mask[0] = 0;
+ val = vcpu->arch.irqs_pending[0] & mask;
csr->hvip &= ~mask;
csr->hvip |= val;
}
+ raw_spin_unlock_irqrestore(&vcpu->arch.irqs_pending_lock, flags);
/* Flush AIA high interrupts */
kvm_riscv_vcpu_aia_flush_interrupts(vcpu);
@@ -368,6 +376,7 @@ void kvm_riscv_vcpu_flush_interrupts(struct kvm_vcpu *vcpu)
void kvm_riscv_vcpu_sync_interrupts(struct kvm_vcpu *vcpu)
{
unsigned long hvip;
+ unsigned long flags;
struct kvm_vcpu_arch *v = &vcpu->arch;
struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
@@ -376,6 +385,7 @@ void kvm_riscv_vcpu_sync_interrupts(struct kvm_vcpu *vcpu)
/* Sync-up HVIP.VSSIP bit changes does by Guest */
hvip = ncsr_read(CSR_HVIP);
+ raw_spin_lock_irqsave(&v->irqs_pending_lock, flags);
if ((csr->hvip ^ hvip) & (1UL << IRQ_VS_SOFT)) {
if (hvip & (1UL << IRQ_VS_SOFT)) {
if (!test_and_set_bit(IRQ_VS_SOFT,
@@ -394,6 +404,7 @@ void kvm_riscv_vcpu_sync_interrupts(struct kvm_vcpu *vcpu)
!test_and_set_bit(IRQ_PMU_OVF, v->irqs_pending_mask))
clear_bit(IRQ_PMU_OVF, v->irqs_pending);
}
+ raw_spin_unlock_irqrestore(&v->irqs_pending_lock, flags);
/* Sync-up AIA high interrupts */
kvm_riscv_vcpu_aia_sync_interrupts(vcpu);
@@ -404,6 +415,8 @@ void kvm_riscv_vcpu_sync_interrupts(struct kvm_vcpu *vcpu)
int kvm_riscv_vcpu_set_interrupt(struct kvm_vcpu *vcpu, unsigned int irq)
{
+ unsigned long flags;
+
/*
* We only allow VS-mode software, timer, and external
* interrupts when irq is one of the local interrupts
@@ -416,9 +429,10 @@ int kvm_riscv_vcpu_set_interrupt(struct kvm_vcpu *vcpu, unsigned int irq)
irq != IRQ_PMU_OVF)
return -EINVAL;
+ raw_spin_lock_irqsave(&vcpu->arch.irqs_pending_lock, flags);
set_bit(irq, vcpu->arch.irqs_pending);
- smp_mb__before_atomic();
set_bit(irq, vcpu->arch.irqs_pending_mask);
+ raw_spin_unlock_irqrestore(&vcpu->arch.irqs_pending_lock, flags);
kvm_vcpu_kick(vcpu);
@@ -427,6 +441,8 @@ int kvm_riscv_vcpu_set_interrupt(struct kvm_vcpu *vcpu, unsigned int irq)
int kvm_riscv_vcpu_unset_interrupt(struct kvm_vcpu *vcpu, unsigned int irq)
{
+ unsigned long flags;
+
/*
* We only allow VS-mode software, timer, counter overflow and external
* interrupts when irq is one of the local interrupts
@@ -439,9 +455,10 @@ int kvm_riscv_vcpu_unset_interrupt(struct kvm_vcpu *vcpu, unsigned int irq)
irq != IRQ_PMU_OVF)
return -EINVAL;
+ raw_spin_lock_irqsave(&vcpu->arch.irqs_pending_lock, flags);
clear_bit(irq, vcpu->arch.irqs_pending);
- smp_mb__before_atomic();
set_bit(irq, vcpu->arch.irqs_pending_mask);
+ raw_spin_unlock_irqrestore(&vcpu->arch.irqs_pending_lock, flags);
return 0;
}
diff --git a/arch/riscv/kvm/vcpu_onereg.c b/arch/riscv/kvm/vcpu_onereg.c
index bb920e892..cba368294 100644
--- a/arch/riscv/kvm/vcpu_onereg.c
+++ b/arch/riscv/kvm/vcpu_onereg.c
@@ -298,6 +298,7 @@ static int kvm_riscv_vcpu_general_set_csr(struct kvm_vcpu *vcpu,
{
struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
unsigned long regs_max = sizeof(struct kvm_riscv_csr) / sizeof(unsigned long);
+ unsigned long flags;
if (reg_num >= regs_max)
return -ENOENT;
@@ -309,10 +310,14 @@ static int kvm_riscv_vcpu_general_set_csr(struct kvm_vcpu *vcpu,
reg_val <<= VSIP_TO_HVIP_SHIFT;
}
- ((unsigned long *)csr)[reg_num] = reg_val;
-
- if (reg_num == KVM_REG_RISCV_CSR_REG(sip))
- WRITE_ONCE(vcpu->arch.irqs_pending_mask[0], 0);
+ if (reg_num == KVM_REG_RISCV_CSR_REG(sip)) {
+ raw_spin_lock_irqsave(&vcpu->arch.irqs_pending_lock, flags);
+ ((unsigned long *)csr)[reg_num] = reg_val;
+ vcpu->arch.irqs_pending_mask[0] = 0;
+ raw_spin_unlock_irqrestore(&vcpu->arch.irqs_pending_lock, flags);
+ } else {
+ ((unsigned long *)csr)[reg_num] = reg_val;
+ }
return 0;
}
base-commit: 481329ec5b31d2c48ac6b8b703c8008133884c7e
--
2.54.0
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