linux-kernel - Re: [PATCH v2] vfio_pci: Replace pci_try_reset_function() with __pci_reset_function_locked() to ensure that the pci device configuration space is restored to its original state

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Date:   Mon, 19 Aug 2019 13:53:18 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     hexin <hexin.op@...il.com>
Cc:     kvm@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, hexin <hexin15@...du.com>,
        Liu Qi <liuqi16@...du.com>, Zhang Yu <zhangyu31@...du.com>
Subject: Re: [PATCH v2] vfio_pci: Replace pci_try_reset_function() with
 __pci_reset_function_locked() to ensure that the pci device configuration
 space is restored to its original state

On Sat, 17 Aug 2019 19:51:03 +0800
hexin <hexin.op@...il.com> wrote:

> In vfio_pci_enable(), save the device's initial configuration information
> and then restore the configuration in vfio_pci_disable(). However, the
> execution result is not the same. Since the pci_try_reset_function()
> function saves the current state before resetting, the configuration
> information restored by pci_load_and_free_saved_state() will be
> overwritten. The __pci_reset_function_locked() function can be used
> to prevent the configuration space from being overwritten.
> 
> Fixes: 890ed578df82 ("vfio-pci: Use pci "try" reset interface")
> Signed-off-by: hexin <hexin15@...du.com>
> Signed-off-by: Liu Qi <liuqi16@...du.com>
> Signed-off-by: Zhang Yu <zhangyu31@...du.com>
> ---
>  drivers/vfio/pci/vfio_pci.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)

This looks good, but the subject is too long and I find the commit log
somewhat confusing.  May I update these as follows?

    vfio_pci: Restore original state on release
    
    vfio_pci_enable() saves the device's initial configuration information
    with the intent that it is restored in vfio_pci_disable().  However,
    commit 890ed578df82 ("vfio-pci: Use pci "try" reset interface")
    replaced the call to __pci_reset_function_locked(), which is not wrapped
    in a state save and restore, with pci_try_reset_function(), which
    overwrites the restored device state with the current state before
    applying it to the device.  Restore use of __pci_reset_function_locked()
    to return to the desired behavior.

Thanks,
Alex


> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index 703948c..0220616 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -438,11 +438,20 @@ static void vfio_pci_disable(struct vfio_pci_device *vdev)
>  	pci_write_config_word(pdev, PCI_COMMAND, PCI_COMMAND_INTX_DISABLE);
>  
>  	/*
> -	 * Try to reset the device.  The success of this is dependent on
> -	 * being able to lock the device, which is not always possible.
> +	 * Try to get the locks ourselves to prevent a deadlock. The
> +	 * success of this is dependent on being able to lock the device,
> +	 * which is not always possible.
> +	 * We can not use the "try" reset interface here, which will
> +	 * overwrite the previously restored configuration information.
>  	 */
> -	if (vdev->reset_works && !pci_try_reset_function(pdev))
> -		vdev->needs_reset = false;
> +	if (vdev->reset_works && pci_cfg_access_trylock(pdev)) {
> +		if (device_trylock(&pdev->dev)) {
> +			if (!__pci_reset_function_locked(pdev))
> +				vdev->needs_reset = false;
> +			device_unlock(&pdev->dev);
> +		}
> +		pci_cfg_access_unlock(pdev);
> +	}
>  
>  	pci_restore_state(pdev);
>  out: