[PATCH RT] vfio-pci: Set MSI/MSI-X ISR to non-threaded

[David Su <david.w.su@intel.com>]

Setting MSI/MSI-X ISR to be non-threaded will result in shorter and more
deterministic IRQ delivery latencies to VFIO applications, because
context switches to the ISR thread are eliminated.  This is important
for applications with low latency requirement running in virtual
machines on RT Linux host with assigned devices through vfio-pci.

A FPGA based interrupt testing device was used to compare latencies with
threaded and non-threaded vfio-pci ISR.  The device has a free running
time stamp counter and a register recording the time an interrupt was
sent to the host.  With these registers the device driver and test
application for the device are able to calculate and record the latency
between the time an interrupt was sent and the time the ISR in the
device's driver was invoked.

The result is with non-threaded vfio-pci ISR the average latency is
reduced by about 54% and the maximum-minimum latency range is reduced by
about 65%.

Non-threaded vfio-pci ISR:
Minimum 4.18us, Average 4.47us, Maximum 10.26us

Threaded vfio-pci ISR:
Minimum 8.97us, Average 9.65us, Maximum 26.11us

Signed-off-by: David Su <david.w.su@intel.com>
---
 drivers/vfio/pci/vfio_pci_intrs.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
index 1c46045..4c54e56 100644
--- a/drivers/vfio/pci/vfio_pci_intrs.c
+++ b/drivers/vfio/pci/vfio_pci_intrs.c
@@ -333,7 +333,7 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_device *vdev,
 		pci_write_msi_msg(irq, &msg);
 	}
 
-	ret = request_irq(irq, vfio_msihandler, 0,
+	ret = request_irq(irq, vfio_msihandler, IRQF_NO_THREAD,
 			  vdev->ctx[vector].name, trigger);
 	if (ret) {
 		kfree(vdev->ctx[vector].name);
-- 
1.7.0.4