linux-kernel - Re: [PATCH v2 1/2] PCI/VPD: Add runtime power management to sysfs interface

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Date:   Thu, 10 Aug 2023 10:26:44 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, eric.auger@...hat.com
Subject: Re: [PATCH v2 1/2] PCI/VPD: Add runtime power management to sysfs
 interface

On Thu, 10 Aug 2023 10:59:26 -0500
Bjorn Helgaas <helgaas@...nel.org> wrote:

> On Thu, Aug 03, 2023 at 11:12:32AM -0600, Alex Williamson wrote:
> > Unlike default access to config space through sysfs, the vpd read and
> > write function don't actively manage the runtime power management state
> > of the device during access.  Since commit 7ab5e10eda02 ("vfio/pci: Move
> > the unused device into low power state with runtime PM"), the vfio-pci
> > driver will use runtime power management and release unused devices to
> > make use of low power states.  Attempting to access VPD information in
> > this low power state can result in incorrect information or kernel
> > crashes depending on the system behavior.
> > 
> > Wrap the vpd read/write bin attribute handlers in runtime PM and take
> > into account the potential quirk to select the correct device to wake.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > ---
> >  drivers/pci/vpd.c | 34 ++++++++++++++++++++++++++++++++--
> >  1 file changed, 32 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/pci/vpd.c b/drivers/pci/vpd.c
> > index a4fc4d0690fe..81217dd4789f 100644
> > --- a/drivers/pci/vpd.c
> > +++ b/drivers/pci/vpd.c
> > @@ -275,8 +275,23 @@ static ssize_t vpd_read(struct file *filp, struct kobject *kobj,
> >  			size_t count)
> >  {
> >  	struct pci_dev *dev = to_pci_dev(kobj_to_dev(kobj));
> > +	struct pci_dev *vpd_dev = dev;
> > +	ssize_t ret;
> > +
> > +	if (dev->dev_flags & PCI_DEV_FLAGS_VPD_REF_F0) {
> > +		vpd_dev = pci_get_func0_dev(dev);
> > +		if (!vpd_dev)
> > +			return -ENODEV;
> > +	}
> > +
> > +	pci_config_pm_runtime_get(vpd_dev);
> > +	ret = pci_read_vpd(vpd_dev, off, count, buf);
> > +	pci_config_pm_runtime_put(vpd_dev);
> > +
> > +	if (dev != vpd_dev)
> > +		pci_dev_put(vpd_dev);  
> 
> I first thought this would leak a reference if dev was func0 and had
> PCI_DEV_FLAGS_VPD_REF_F0 set, because in that case vpd_dev would be
> the same as dev.
> 
> But I think that case can't happen because quirk_f0_vpd_link() does
> nothing for func0 devices, so PCI_DEV_FLAGS_VPD_REF_F0 should never be
> set for func0.  But it seems like this might be easier to analyze as:
> 
>   if (dev->dev_flags & PCI_DEV_FLAGS_VPD_REF_F0)
>     pci_dev_put(vpd_dev);
> 
> Or am I missing something?

Nope, your analysis is correct, it doesn't make any sense to have a
flag on func0 redirecting VPD access to func0 so vpd_dev can only be
different on non-zero functions.  The alternative test is equally
valid so if you think it's more intuitive, let's use it.  Thanks,

Alex