On Mon, 2015-10-26 at 18:20 -0700, Yunhong Jiang wrote:
> 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.
Is there any way we can do this automatically? Perhaps detecting that
we're on a RT kernel or maybe that the user is running with RT priority?
I find that module options are mostly misunderstood and misused.
> 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.
Allowing the user to decide whether or not to use a threaded IRQ seems
like a privilege violation; a chance for the user to game the system and
give themselves better latency, maybe at the cost of others. I think
we're better off trying to infer the privilege from the task priority or
kernel config or, if we run out of options, make a module option as you
have here requiring the system admin to provide the privilege. Thanks,
Alex
> 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) {
> kfree(vdev->ctx[vector].name);
--
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