Re: [PATCH v3 6/8] vfio: Invoke runtime PM API for IOCTL request

Alex Williamson
2022-05-04T19:44:05Z

On Mon, 25 Apr 2022 14:56:13 +0530
Abhishek Sahu <abhsahu@nvidia.com> wrote:

> The vfio/pci driver will have runtime power management support where the
> user can put the device low power state and then PCI devices can go into
> the D3cold state. If the device is in low power state and user issues any
> IOCTL, then the device should be moved out of low power state first. Once
> the IOCTL is serviced, then it can go into low power state again. The
> runtime PM framework manages this with help of usage count. One option
> was to add the runtime PM related API's inside vfio/pci driver but some
> IOCTL (like VFIO_DEVICE_FEATURE) can follow a different path and more
> IOCTL can be added in the future. Also, the runtime PM will be
> added for vfio/pci based drivers variant currently but the other vfio
> based drivers can use the same in the future. So, this patch adds the
> runtime calls runtime related API in the top level IOCTL function itself.
>
> For the vfio drivers which do not have runtime power management support
> currently, the runtime PM API's won't be invoked. Only for vfio/pci
> based drivers currently, the runtime PM API's will be invoked to increment
> and decrement the usage count. Taking this usage count incremented while
> servicing IOCTL will make sure that user won't put the device into low
> power state when any other IOCTL is being serviced in parallel.
>
> Signed-off-by: Abhishek Sahu <abhsahu@nvidia.com>
> ---
> drivers/vfio/vfio.c | 44 +++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 41 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index a4555014bd1e..4e65a127744e 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -32,6 +32,7 @@
> #include <linux/vfio.h>
> #include <linux/wait.h>
> #include <linux/sched/signal.h>
> +#include <linux/pm_runtime.h>
> #include "vfio.h"
>
> #define DRIVER_VERSION "0.3"
> @@ -1536,6 +1537,30 @@ static const struct file_operations vfio_group_fops = {
> .release = vfio_group_fops_release,
> };
>
> +/*
> + * Wrapper around pm_runtime_resume_and_get().
> + * Return 0, if driver power management callbacks are not present i.e. the driver is not

Mind the gratuitous long comment line here.

> + * using runtime power management.
> + * Return 1 upon success, otherwise -errno

Changing semantics vs the thing we're wrapping, why not provide a
wrapper for the `put` as well to avoid? The only cases where we return
zero are just as easy to detect on the other side.

> + */
> +static inline int vfio_device_pm_runtime_get(struct device *dev)

Given some of Jason's recent series, this should probably just accept a
vfio_device.

> +{
> +#ifdef CONFIG_PM
> + int ret;
> +
> + if (!dev->driver || !dev->driver->pm)
> + return 0;
> +
> + ret = pm_runtime_resume_and_get(dev);
> + if (ret < 0)
> + return ret;
> +
> + return 1;
> +#else
> + return 0;
> +#endif
> +}
> +
> /*
> * VFIO Device fd
> */
> @@ -1845,15 +1870,28 @@ static long vfio_device_fops_unl_ioctl(struct file *filep,
> unsigned int cmd, unsigned long arg)
> {
> struct vfio_device *device = filep->private_data;
> + int pm_ret, ret = 0;
> +
> + pm_ret = vfio_device_pm_runtime_get(device->dev);
> + if (pm_ret < 0)
> + return pm_ret;

I wonder if we might simply want to mask pm errors behind -EIO, maybe
with a rate limited dev_info(). My concern would be that we might mask
errnos that userspace has come to expect for certain ioctls. Thanks,

Alex

>
> switch (cmd) {
> case VFIO_DEVICE_FEATURE:
> - return vfio_ioctl_device_feature(device, (void __user *)arg);
> + ret = vfio_ioctl_device_feature(device, (void __user *)arg);
> + break;
> default:
> if (unlikely(!device->ops->ioctl))
> - return -EINVAL;
> - return device->ops->ioctl(device, cmd, arg);
> + ret = -EINVAL;
> + else
> + ret = device->ops->ioctl(device, cmd, arg);
> + break;
> }
> +
> + if (pm_ret)
> + pm_runtime_put(device->dev);
> +
> + return ret;
> }
>
> static ssize_t vfio_device_fops_read(struct file *filep, char __user *buf,