Hi Peter,
On 9/23/22 7:38 AM, Peter Xu wrote:
On Thu, Sep 22, 2022 at 06:01:28PM +0100, Marc Zyngier wrote:
The current implementation of the dirty ring has an implicit requirement
that stores to the dirty ring from userspace must be:
- be ordered with one another
- visible from another CPU executing a ring reset
While these implicit requirements work well for x86 (and any other
TSO-like architecture), they do not work for more relaxed architectures
such as arm64 where stores to different addresses can be freely
reordered, and loads from these addresses not observing writes from
another CPU unless the required barriers (or acquire/release semantics)
are used.
In order to start fixing this, upgrade the ring reset accesses:
- the kvm_dirty_gfn_harvested() helper now uses acquire semantics
so it is ordered after all previous writes, including that from
userspace
- the kvm_dirty_gfn_set_invalid() helper now uses release semantics
so that the next_slot and next_offset reads don't drift past
the entry invalidation
This is only a partial fix as the userspace side also need upgrading.
Paolo has one fix 4802bf910e ("KVM: dirty ring: add missing memory
barrier", 2022-09-01) which has already landed.
I think the other one to reset it was lost too. I just posted a patch.
https://lore.kernel.org/qemu-devel/20220922213522.68861-1-peterx@xxxxxxxxxx/
(link still not yet available so far, but should be)
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
---
virt/kvm/dirty_ring.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index f4c2a6eb1666..784bed80221d 100644
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -79,12 +79,12 @@ static inline void kvm_dirty_gfn_set_invalid(struct kvm_dirty_gfn *gfn)
static inline void kvm_dirty_gfn_set_dirtied(struct kvm_dirty_gfn *gfn)
{
- gfn->flags = KVM_DIRTY_GFN_F_DIRTY;
+ smp_store_release(&gfn->flags, KVM_DIRTY_GFN_F_DIRTY);
IIUC you meant kvm_dirty_gfn_set_invalid as the comment says?
kvm_dirty_gfn_set_dirtied() has been guarded by smp_wmb() and AFAICT that's
already safe. Otherwise looks good to me.
If I'm understanding the full context, smp_store_release() also enforces
guard on 'gfn->flags' itself. It is needed by user space for the synchronization.
}
static inline bool kvm_dirty_gfn_harvested(struct kvm_dirty_gfn *gfn)
{
- return gfn->flags & KVM_DIRTY_GFN_F_RESET;
+ return smp_load_acquire(&gfn->flags) & KVM_DIRTY_GFN_F_RESET;
}
int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
--
2.34.1
Thanks,
Gavin
