On 30/10/2020 10:49, Jan Beulich wrote:
On 30.10.2020 11:38, Julien Grall wrote:
On 22/10/2020 17:17, Jan Beulich wrote:
On 22.10.2020 18:00, Roger Pau Monné wrote:
On Tue, Oct 20, 2020 at 04:10:09PM +0200, Jan Beulich wrote:
--- a/xen/include/xen/event.h
+++ b/xen/include/xen/event.h
@@ -177,9 +177,16 @@ int evtchn_reset(struct domain *d, bool
* Low-level event channel port ops.
*
* All hooks have to be called with a lock held which prevents the channel
- * from changing state. This may be the domain event lock, the per-channel
- * lock, or in the case of sending interdomain events also the other side's
- * per-channel lock. Exceptions apply in certain cases for the PV shim.
+ * from changing state. This may be
+ * - the domain event lock,
+ * - the per-channel lock,
+ * - in the case of sending interdomain events the other side's per-channel
+ * lock,
+ * - in the case of sending non-global vIRQ-s the per-vCPU virq_lock (in
+ * combination with the ordering enforced through how the vCPU's
+ * virq_to_evtchn[] gets updated),
+ * - in the case of sending global vIRQ-s vCPU 0's virq_lock.
+ * Exceptions apply in certain cases for the PV shim.
Having such a wide locking discipline looks dangerous to me, it's easy
to get things wrong without notice IMO.
It is effectively only describing how things are (or were before
XSA-343, getting restored here).
I agree with Roger here, the new/old locking discipline is dangerous and
it is only a matter of time before it will bite us again.
I think we should consider Juergen's series because the locking for the
event channel is easier to understand.
We should, yes. The one thing I'm a little uneasy with is the
new lock "variant" that gets introduced. Custom locking methods
also are a common source of problems (which isn't to say I see
any here).
I am also unease with a new lock "variant". However, this is the best
proposal I have seen so far to unblock the issue.
I am open to other suggestion with simple locking discipline.
With his series in place, this patch will become unecessary.
It'll become less important, but not pointless - any unnecessary
locking would better be removed imo.
They may be unnecessary today but if tomorrow someone decide to rework
the other lock, then you are just re-opening a security hole.
IHMO, having a sane locking system is far more important than removing
locking that look "unnecessary".
I'd also like to note that the non-straightforward locking rules
wouldn't really change with his series; the benefit there really
is the dropping of the need for IRQ-safe locking.
Well, it is at least going towards that...
Cheers,
Jan
--
Julien Grall
