Date:   Thu, 21 Jul 2022 16:34:51 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Abhishek Sahu <abhsahu@...dia.com>
Cc:     Cornelia Huck <cohuck@...hat.com>,
        Yishai Hadas <yishaih@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
        Kevin Tian <kevin.tian@...el.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Max Gurtovoy <mgurtovoy@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <linux-pm@...r.kernel.org>, <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v5 2/5] vfio: Increment the runtime PM usage count
 during IOCTL call

On Tue, 19 Jul 2022 17:45:20 +0530
Abhishek Sahu <abhsahu@...dia.com> wrote:

> The vfio-pci based drivers 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. In the vfio-pci drivers also,
> the variant drivers can opt-out by incrementing the usage count during
> device-open. The pm_runtime_resume_and_get() checks the device
> current status and will return early if the device is already in the
> ACTIVE state.
> 
> Taking this usage count incremented while servicing IOCTL will make
> sure that the user won't put the device into the 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 IOCTL for
>     low power entry.
>  3. The low power entry 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 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@...dia.com>
> ---
>  drivers/vfio/vfio.c | 52 ++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 49 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index bd84ca7c5e35..1d005a0a9d3d 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"
> @@ -1335,6 +1336,39 @@ 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) {

Nit, pm_runtime_resume_and_get() cannot return a positive value, it's
either zero or -errno, so we could just test (ret).  Thanks,

Alex

> +			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
>   */
> @@ -1649,15 +1683,27 @@ 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;
> +
> +	ret = vfio_device_pm_runtime_get(device);
> +	if (ret)
> +		return ret;
>  
>  	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;
>  	}
> +
> +	vfio_device_pm_runtime_put(device);
> +	return ret;
>  }
>  
>  static ssize_t vfio_device_fops_read(struct file *filep, char __user *buf,

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