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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Message-ID: <20190821103917.43487a1d@x1.home>
Date:   Wed, 21 Aug 2019 10:39:17 -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 Wed, 21 Aug 2019 23:13:08 +0800
hexin <hexin.op@...il.com> wrote:

> Alex Williamson <alex.williamson@...hat.com> 于2019年8月20日周二 上午3:53写道：
> >
> > 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
> >
> >  
> 
> Thanks for your update, the updated commit log is clearer than before.
> At the same time, when I use checkpatch.pl to detect the patch, there
> will be the
> following error:
> 
> ERROR: Please use git commit description style 'commit <12+ chars of
> sha1> ("<title line>")'  
> - ie: 'commit 890ed578df82 ("vfio-pci: Use pci "try" reset interface")'
> 
> Line 2785 ~ 2801 in checkpatch.pl, the script can't handle the commit message
> which contains double quotes because of the expression `([^"]+)`. Like
> the "try" above.
> Maybe checkpatch.pl needs to be modified.

I think we're following the intention of the rule, and as you've
identified it's the implementation of the rule checker that's unable
to handle a commit title with internal quotes.  We can ignore it, and
maybe follow up with a checkpatch.pl patch, or we could just avoid it
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,
    the commit referenced in Fixes: below 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.  Reinstate 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:  
> >