[Xen-devel] passthrough: improve interrupt injection locking

[David Vrabel <david.vrabel@xxxxxxxxxx>]

When injecting an interrupt for a passthrough device into a guest, the
per-domain event_lock is held, reducing performance when a guest has
many VCPUs and high interrupt rates.

By using a per-interrupt lock in the hot paths, this contention is
eliminated and performance improves (a bit).

For testing, a 32 VCPU guest with an NVME device assigned to it was
used.  Continual reads with small (512 B) blocks were performed on all
32 hardware queues simultaneously.

* Lock profiling:

Before (elapsed: 60 s):

(XEN) [ 3321.143155] Domain 1 event_lock:
(XEN) [ 3321.143158]   lock:    14411627(00000005:90714AEF),
                      block:     6658599(00000003:709F82BD)

After (elapsed: 60 s):

(XEN) [ 1253.921427] Domain 2 event_lock:
(XEN) [ 1253.921429]   lock:        8287(00000000:01AE517C),
                      block:          67(00000000:000D4C3A)

* Aggregate performance:

        MB/s
Before  60.8
After   68.4

David


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