MFC 302181,302635: Disable MSI-X migration on older Xen hypervisors.

302181:
Add a tunable to disable migration of MSI-X interrupts.

The new 'machdep.disable_msix_migration' tunable can be set to 1 to
disable migration of MSI-X interrupts.

Xen versions prior to 4.6.0 do not properly handle updates to MSI-X
table entries after the initial write.  In particular, the operation
to unmask a table entry after updating it during migration is not
propagated to the "real" table for passthrough devices causing the
interrupt to remain masked.  At least some systems in EC2 are
affected by this bug when using SRIOV.  The tunable can be set in
loader.conf as a workaround.

302635:
xen: automatically disable MSI-X interrupt migration

If the hypervisor version is smaller than 4.6.0. Xen commits 74fd00 and
70a3cb are required on the hypervisor side for this to be fixed, and those
are only included in 4.6.0, so stay on the safe side and disable MSI-X
interrupt migration on anything older than 4.6.0.

It should not cause major performance degradation unless a lot of MSI-X
interrupts are allocated.

1 files changed, 2 insertions, 0 deletions

diff --git a/sys/i386/include/intr_machdep.h b/sys/i386/include/intr_machdep.h
index 8fb61a5..b71bbb6 100644
--- a/sys/i386/include/intr_machdep.h
+++ b/sys/i386/include/intr_machdep.h
@@ -141,6 +141,8 @@ struct trapframe;
 extern struct mtx icu_lock;
 extern int elcr_found;
 
+extern int msix_disable_migration;
+
 #ifndef DEV_ATPIC
 void	atpic_reset(void);
 #endif