linux-kernel - Re: [Xen-devel] [PATCH v5] Fixes to Xen pciback for 3.17.

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Date:	Thu, 7 Aug 2014 10:04:02 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Sander Eikelenboom <linux@...elenboom.it>
CC:	<gregkh@...uxfoundation.org>, <boris.ostrovsky@...cle.com>,
	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>
Subject: Re: [Xen-devel] [PATCH v5] Fixes to Xen pciback for 3.17.

On 06/08/14 20:39, Konrad Rzeszutek Wilk wrote:
> 
> From 00a5b6e3c9ee2c2d605879bdaebc627fa640b024 Mon Sep 17 00:00:00 2001
> From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Date: Wed, 6 Aug 2014 16:21:32 -0400
> Subject: [PATCH] xen/pciback: Restore configuration space when detaching from
>  a guest.
> 
> The commit 9eea3f7695226f9af9992cebf8e98ac0ad78b277
> "xen/pciback: Don't deadlock when unbinding." was using
> the version of pci_reset_function which would lock the device lock.
> That is no good as we can dead-lock. As such we swapped to using
> the lock-less version and requiring that the callers
> of 'pcistub_put_pci_dev' take the device lock. And as such
> this bug got exposed.
> 
> Using the lock-less version is  OK, except that we tried to
> use 'pci_restore_state' after the lock-less version of
> __pci_reset_function_locked - which won't work as 'state_saved'
> is set to false. Said 'state_saved' is a toggle boolean that
> is to be used by the sequence of a) pci_save_state/pci_restore_state
> or b) pci_load_and_free_saved_state/pci_restore_state. We don't
> want to use a) as the guest might have messed up the PCI
> configuration space and we want it to revert to the state
> when the PCI device was binded to us. Therefore we pick
> b) to restore the configuration space.
> 
> To still retain the PCI configuration space, we save it once
> more and store it on our private copy to be restored when:
>  - Device is unbinded from pciback
>  - Device is detached from a guest.

This should be folded into the original patch.

[...]
> --- a/drivers/xen/xen-pciback/pci_stub.c
> +++ b/drivers/xen/xen-pciback/pci_stub.c
> @@ -105,7 +105,7 @@ static void pcistub_device_release(struct kref *kref)
>  	 */
>  	__pci_reset_function_locked(dev);
>  	if (pci_load_and_free_saved_state(dev, &dev_data->pci_saved_state))

I dislike testing for errors like this as it looks like it's testing for
a boolean success.  Use

   ret = pci_load_and_free_saved_state(...);
   if (ret < 0)
      ...

And similarly, below.

> -		dev_dbg(&dev->dev, "Could not reload PCI state\n");
> +		dev_info(&dev->dev, "Could not reload PCI state\n");

This should be dev_warn().

pci_load_and_free_saved_state() won't fail because we know the state is
valid (since we saved it from the device originally).  Warning and
skipping the restore is fine here (and below).

>  	else
>  		pci_restore_state(dev);
>  
> @@ -257,6 +257,7 @@ void pcistub_put_pci_dev(struct pci_dev *dev)
>  {
>  	struct pcistub_device *psdev, *found_psdev = NULL;
>  	unsigned long flags;
> +	struct xen_pcibk_dev_data *dev_data;
>  
>  	spin_lock_irqsave(&pcistub_devices_lock, flags);
>  
> @@ -279,9 +280,25 @@ void pcistub_put_pci_dev(struct pci_dev *dev)
>  	 * (so it's ready for the next domain)
>  	 */
>  	device_lock_assert(&dev->dev);
> -	__pci_reset_function_locked(dev);
> -	pci_restore_state(dev);
> -
> +	dev_data = pci_get_drvdata(dev);
> +	if (pci_load_and_free_saved_state(dev, &dev_data->pci_saved_state))

This should be pci_load_saved_state() and then you can avoid the
pci_save_state() below.

> +		dev_info(&dev->dev, "Could not reload PCI state\n");

dev_warn() also.

> +	else {
> +		__pci_reset_function_locked(dev);
> +		/*
> +		 * The usual sequence is pci_save_state & pci_restore_state
> +		 * but the guest might have messed the configuration space up.
> +		 * Use the initial version (when device was binded to us).

s/binded/bound/

> +		pci_restore_state(dev);
> +		/*
> +		 * The next steps are to reload the configuration for the
> +		 * next time we bind & unbind to a guest - or unload from
> +		 * pciback.
> +		 */
> +		pci_save_state(dev);
> +		dev_data->pci_saved_state = pci_store_saved_state(dev);

You don't need this if you don't free the original state above.

> +	}
>  	/* This disables the device. */
>  	xen_pcibk_reset_device(dev);

David
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