[RFC PATCH] VFIO: Add a parameter to force nonthread IRQ

From: Yunhong Jiang
Date: Mon Oct 26 2015 - 21:31:03 EST

An option to force VFIO PCI MSI/MSI-X handler as non-threaded IRQ,
even when CONFIG_IRQ_FORCED_THREADING=y. This is uselful when
assigning a device to a guest with low latency requirement since it
reduce the context switch to/from the IRQ thread.

An experiment was conducted on a HSW platform for 1 minutes, with the
guest vCPU bound to isolated pCPU. The assigned device triggered the
interrupt every 1ms. The average EXTERNAL_INTERRUPT exit handling time
is dropped from 5.3us to 2.2us.

Another choice is to change VFIO_DEVICE_SET_IRQS ioctl, to apply this
option only to specific devices when in kernel irq_chip is enabled. It
provides more flexibility but is more complex, not sure if we need go
through that way.

Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx>
drivers/vfio/pci/vfio_pci_intrs.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
index 1f577b4..ca1f95a 100644
--- a/drivers/vfio/pci/vfio_pci_intrs.c
+++ b/drivers/vfio/pci/vfio_pci_intrs.c
@@ -22,9 +22,13 @@
#include <linux/vfio.h>
#include <linux/wait.h>
#include <linux/slab.h>
+#include <linux/module.h>

#include "vfio_pci_private.h"

+static bool nonthread_msi = 1;
+module_param(nonthread_msi, bool, 0444);
* INTx
@@ -313,6 +317,7 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_device *vdev,
char *name = msix ? "vfio-msix" : "vfio-msi";
struct eventfd_ctx *trigger;
int ret;
+ unsigned long irqflags = 0;

if (vector >= vdev->num_ctx)
return -EINVAL;
@@ -352,7 +357,10 @@ 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,
+ if (nonthread_msi)
+ irqflags = IRQF_NO_THREAD;
+ ret = request_irq(irq, vfio_msihandler, irqflags,
vdev->ctx[vector].name, trigger);
if (ret) {

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