Re: [PATCH v4 3/6] vfio: Increment the runtime PM usage count during IOCTL call

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 1 Jul 2022 16:38:11 +0530
Abhishek Sahu <abhsahu@nvidia.com> wrote:

> The vfio-pci based driver will have runtime power management
> support where the user can put the device into the low power state
> and then PCI devices can go into the D3cold state. If the device is
> in the low power state and the user issues any IOCTL, then the
> device should be moved out of the low power state first. Once
> the IOCTL is serviced, then it can go into the 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.

Variant drivers can easily opt-out of runtime pm support by performing
a gratuitous pm-get in their device-open function.
 
> Taking this usage count incremented while servicing IOCTL will make
> sure that the user won't put the device into low power state when any
> other IOCTL is being serviced in parallel. Let's consider the
> following scenario:
> 
>  1. Some other IOCTL is called.
>  2. The user has opened another device instance and called the power
>     management IOCTL for the low power entry.
>  3. The power management IOCTL moves the device into the low power state.
>  4. The other IOCTL finishes.
> 
> If we don't keep the usage count incremented then the device
> access will happen between step 3 and 4 while the device has already
> gone into the low power state.
> 
> The runtime PM API's should not be invoked for
> VFIO_DEVICE_FEATURE_POWER_MANAGEMENT since this IOCTL itself performs
> the runtime power management entry and exit for the VFIO device.

I think the one-shot interface I proposed in the previous patch avoids
the need for special handling for these feature ioctls.  Thanks,

Alex
 
> The pm_runtime_resume_and_get() will be the first call so its error
> should not be propagated to user space directly. For example, if
> pm_runtime_resume_and_get() can return -EINVAL for the cases where the
> user has passed the correct argument. So the
> pm_runtime_resume_and_get() errors have been masked behind -EIO.
> 
> Signed-off-by: Abhishek Sahu <abhsahu@nvidia.com>
> ---
>  drivers/vfio/vfio.c | 82 ++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 74 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index 61e71c1154be..61a8d9f7629a 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"
> @@ -1333,6 +1334,39 @@ static const struct file_operations vfio_group_fops = {
>  	.release	= vfio_group_fops_release,
>  };
>  
> +/*
> + * Wrapper around pm_runtime_resume_and_get().
> + * Return error code on failure or 0 on success.
> + */
> +static inline int vfio_device_pm_runtime_get(struct vfio_device *device)
> +{
> +	struct device *dev = device->dev;
> +
> +	if (dev->driver && dev->driver->pm) {
> +		int ret;
> +
> +		ret = pm_runtime_resume_and_get(dev);
> +		if (ret < 0) {
> +			dev_info_ratelimited(dev,
> +				"vfio: runtime resume failed %d\n", ret);
> +			return -EIO;
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +/*
> + * Wrapper around pm_runtime_put().
> + */
> +static inline void vfio_device_pm_runtime_put(struct vfio_device *device)
> +{
> +	struct device *dev = device->dev;
> +
> +	if (dev->driver && dev->driver->pm)
> +		pm_runtime_put(dev);
> +}
> +
>  /*
>   * VFIO Device fd
>   */
> @@ -1607,6 +1641,8 @@ static int vfio_ioctl_device_feature(struct vfio_device *device,
>  {
>  	size_t minsz = offsetofend(struct vfio_device_feature, flags);
>  	struct vfio_device_feature feature;
> +	int ret = 0;
> +	u16 feature_cmd;
>  
>  	if (copy_from_user(&feature, arg, minsz))
>  		return -EFAULT;
> @@ -1626,28 +1662,51 @@ static int vfio_ioctl_device_feature(struct vfio_device *device,
>  	    (feature.flags & VFIO_DEVICE_FEATURE_GET))
>  		return -EINVAL;
>  
> -	switch (feature.flags & VFIO_DEVICE_FEATURE_MASK) {
> +	feature_cmd = feature.flags & VFIO_DEVICE_FEATURE_MASK;
> +
> +	/*
> +	 * The VFIO_DEVICE_FEATURE_POWER_MANAGEMENT itself performs the runtime
> +	 * power management entry and exit for the VFIO device, so the runtime
> +	 * PM API's should not be called for this feature.
> +	 */
> +	if (feature_cmd != VFIO_DEVICE_FEATURE_POWER_MANAGEMENT) {
> +		ret = vfio_device_pm_runtime_get(device);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	switch (feature_cmd) {
>  	case VFIO_DEVICE_FEATURE_MIGRATION:
> -		return vfio_ioctl_device_feature_migration(
> +		ret = vfio_ioctl_device_feature_migration(
>  			device, feature.flags, arg->data,
>  			feature.argsz - minsz);
> +		break;
>  	case VFIO_DEVICE_FEATURE_MIG_DEVICE_STATE:
> -		return vfio_ioctl_device_feature_mig_device_state(
> +		ret = vfio_ioctl_device_feature_mig_device_state(
>  			device, feature.flags, arg->data,
>  			feature.argsz - minsz);
> +		break;
>  	default:
>  		if (unlikely(!device->ops->device_feature))
> -			return -EINVAL;
> -		return device->ops->device_feature(device, feature.flags,
> -						   arg->data,
> -						   feature.argsz - minsz);
> +			ret = -EINVAL;
> +		else
> +			ret = device->ops->device_feature(
> +				device, feature.flags, arg->data,
> +				feature.argsz - minsz);
> +		break;
>  	}
> +
> +	if (feature_cmd != VFIO_DEVICE_FEATURE_POWER_MANAGEMENT)
> +		vfio_device_pm_runtime_put(device);
> +
> +	return ret;
>  }
>  
>  static long vfio_device_fops_unl_ioctl(struct file *filep,
>  				       unsigned int cmd, unsigned long arg)
>  {
>  	struct vfio_device *device = filep->private_data;
> +	int ret;
>  
>  	switch (cmd) {
>  	case VFIO_DEVICE_FEATURE:
> @@ -1655,7 +1714,14 @@ static long vfio_device_fops_unl_ioctl(struct file *filep,
>  	default:
>  		if (unlikely(!device->ops->ioctl))
>  			return -EINVAL;
> -		return device->ops->ioctl(device, cmd, arg);
> +
> +		ret = vfio_device_pm_runtime_get(device);
> +		if (ret)
> +			return ret;
> +
> +		ret = device->ops->ioctl(device, cmd, arg);
> +		vfio_device_pm_runtime_put(device);
> +		return ret;
>  	}
>  }
>