Fold coalesced_mmio_has_room() into its sole caller, coalesced_mmio_write(),
as it's really just a single line of code, has a goofy return value, and
is unnecessarily brittle.
E.g. if coalesced_mmio_has_room() were to check ring->last directly, or
the caller failed to use READ_ONCE(), KVM would be susceptible to TOCTOU
attacks from userspace.
Opportunistically add a comment explaining why on earth KVM leaves one
entry free, which may not be obvious to readers that aren't famailiar with
ring buffers.
No functional change intended.
Cc: Ilias Stamatis <ilstam@xxxxxxxxxx>
Cc: Paul Durrant <paul@xxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
virt/kvm/coalesced_mmio.c | 29 ++++++++---------------------
1 file changed, 8 insertions(+), 21 deletions(-)
diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
index 184c5c40c9c1..375d6285475e 100644
--- a/virt/kvm/coalesced_mmio.c
+++ b/virt/kvm/coalesced_mmio.c
@@ -40,25 +40,6 @@ static int coalesced_mmio_in_range(struct kvm_coalesced_mmio_dev *dev,
return 1;
}
-static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev, u32 last)
-{
- struct kvm_coalesced_mmio_ring *ring;
-
- /* Are we able to batch it ? */
-
- /* last is the first free entry
- * check if we don't meet the first used entry
- * there is always one unused entry in the buffer
- */
- ring = dev->kvm->coalesced_mmio_ring;
- if ((last + 1) % KVM_COALESCED_MMIO_MAX == READ_ONCE(ring->first)) {
- /* full */
- return 0;
- }
-
- return 1;
-}
-
static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
struct kvm_io_device *this, gpa_t addr,
int len, const void *val)
@@ -72,9 +53,15 @@ static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
spin_lock(&dev->kvm->ring_lock);
+ /*
+ * last is the index of the entry to fill. Verify userspace hasn't
+ * set last to be out of range, and that there is room in the ring.
+ * Leave one entry free in the ring so that userspace can differentiate
+ * between an empty ring and a full ring.
+ */
insert = READ_ONCE(ring->last);
- if (!coalesced_mmio_has_room(dev, insert) ||
- insert >= KVM_COALESCED_MMIO_MAX) {
+ if (insert >= KVM_COALESCED_MMIO_MAX ||
+ (insert + 1) % KVM_COALESCED_MMIO_MAX == READ_ONCE(ring->first)) {
spin_unlock(&dev->kvm->ring_lock);
return -EOPNOTSUPP;
}
--
2.46.0.295.g3b9ea8a38a-goog
