There are two problems with the current implementation of the MMIO
handlers for the propbaser and pendbaser:
First, the write to the value itself is not guaranteed to be an atomic
64-bit write so two concurrent writes to the structure field could be
intermixed.
Second, because we do a read-modify-update operation without any
synchronization, if we have two 32-bit accesses to separate parts of the
register, we can loose one of them.
By using the atomic cmpxchg64 we should cover both issues above.
Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
---
Notes:
Changes since v1:
- Use atomic cmpxchg64 instead of taking a lock
virt/kvm/arm/vgic/vgic-mmio-v3.c | 24 ++++++++++++++----------
1 file changed, 14 insertions(+), 10 deletions(-)
diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c b/virt/kvm/arm/vgic/vgic-mmio-v3.c
index ff668e0..a50d5ba 100644
--- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
+++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
@@ -306,16 +306,18 @@ static void vgic_mmio_write_propbase(struct kvm_vcpu *vcpu,
{
struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
- u64 propbaser = dist->propbaser;
+ u64 old_propbaser, propbaser;
/* Storing a value with LPIs already enabled is undefined */
if (vgic_cpu->lpis_enabled)
return;
- propbaser = update_64bit_reg(propbaser, addr & 4, len, val);
- propbaser = vgic_sanitise_propbaser(propbaser);
-
- dist->propbaser = propbaser;
+ do {
+ old_propbaser = dist->propbaser;
+ propbaser = old_propbaser;
+ propbaser = update_64bit_reg(propbaser, addr & 4, len, val);
+ propbaser = vgic_sanitise_propbaser(propbaser);
+ } while (cmpxchg64(&dist->propbaser, old_propbaser, propbaser));
}
static unsigned long vgic_mmio_read_pendbase(struct kvm_vcpu *vcpu,
@@ -331,16 +333,18 @@ static void vgic_mmio_write_pendbase(struct kvm_vcpu *vcpu,
unsigned long val)
{
struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
- u64 pendbaser = vgic_cpu->pendbaser;
+ u64 old_pendbaser, pendbaser;
/* Storing a value with LPIs already enabled is undefined */
if (vgic_cpu->lpis_enabled)
return;
- pendbaser = update_64bit_reg(pendbaser, addr & 4, len, val);
- pendbaser = vgic_sanitise_pendbaser(pendbaser);
-
- vgic_cpu->pendbaser = pendbaser;
+ do {
+ old_pendbaser = vgic_cpu->pendbaser;
+ pendbaser = old_pendbaser;
+ pendbaser = update_64bit_reg(pendbaser, addr & 4, len, val);
+ pendbaser = vgic_sanitise_pendbaser(pendbaser);
+ } while (cmpxchg64(&vgic_cpu->pendbaser, old_pendbaser, pendbaser));
}
/*
--
2.9.0
--
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