RE: [ PATCH RESEND ] PCI-AER: Do not report successful error recovery for devices with AER-unaware drivers

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Thanks for the comments. See my response below.

> -----Original Message-----
> From: Bjorn Helgaas [mailto:bhelgaas@xxxxxxxxxx]
> Sent: Wednesday, November 14, 2012 4:51 PM
> To: Pandarathil, Vijaymohan R
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx;
> linasvepstas@xxxxxxxxx; Myron Stowe; Ortiz, Lance E; Huang Ying; Hidetoshi
> Seto; Patterson, Andrew D (LeftHand Networks); Zhang Yanmin
> Subject: Re: [ PATCH RESEND ] PCI-AER: Do not report successful error
> recovery for devices with AER-unaware drivers
> 
> [+cc Lance, Huang, Hidetoshi, Andrew, Zhang]
> 
> On Sat, Nov 10, 2012 at 07:41:04AM +0000, Pandarathil, Vijaymohan R wrote:
> > When an error is detected on a PCIe device which does not have an
> > AER-aware driver, prevent AER infrastructure from reporting
> > successful error recovery.
> >
> > This is because the report_error_detected() function that gets
> > called in the first phase of recovery process allows forward
> > progress even when the driver for the device does not have AER
> > capabilities. It seems that all callbacks (in pci_error_handlers
> > structure) registered by drivers that gets called during error
> > recovery are not mandatory. So the intention of the infrastructure
> > design seems to be to allow forward progress even when a specific
> > callback has not been registered by a driver. However, if error
> > handler structure itself has not been registered, it doesn't make
> > sense to allow forward progress.
> >
> > As a result of the current design, in the case of a single device
> > having an AER-unaware driver or in the case of any function in a
> > multi-function card having an AER-unaware driver, a successful
> > recovery is reported.
> >
> > Typical scenario this happens is when a PCI device is detached
> > from a KVM host and the pci-stub driver on the host claims the
> > device. The pci-stub driver does not have error handling capabilities
> > but the AER infrastructure still reports that the device recovered
> > successfully.
> >
> > The changes proposed here leaves the device in an unrecovered state
> > if the driver for the device or for any function in a multi-function
> > card does not have error handler structure registered. This reflects
> > the true state of the device and prevents any partial recovery (or no
> > recovery at all) reported as successful.
> >
> > Please also see comments from Linas Vepstas at the following link
> > http://www.spinics.net/lists/linux-pci/msg18572.html
> >
> > Reviewed-by: Linas Vepstas <linasvepstas <at> gmail.com>
> > Reviewed-by: Myron Stowe <mstowe <at> redhat.com>
> > Signed-off-by: Vijay Mohan Pandarathil <vijaymohan.pandarathil <at>
> hp.com>
> >
> > ---
> >
> > drivers/pci/pcie/aer/aerdrv_core.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/pci/pcie/aer/aerdrv_core.c
> b/drivers/pci/pcie/aer/aerdrv_core.c
> > index 06bad96..030b229 100644
> > --- a/drivers/pci/pcie/aer/aerdrv_core.c
> > +++ b/drivers/pci/pcie/aer/aerdrv_core.c
> > @@ -215,6 +215,12 @@ static int report_error_detected(struct pci_dev
> *dev, void *data)
> >
> >  	dev->error_state = result_data->state;
> >
> > +	if ((!dev->driver || !dev->driver->err_handler) &&
> > +		!(dev->hdr_type & PCI_HEADER_TYPE_BRIDGE)) {
> > +		dev_info(&dev->dev, "AER: Error detected but no driver has
> claimed this device or the driver is AER-unaware\n");
> > +		result_data->result = PCI_ERS_RESULT_NONE;
> > +		return 1;
> 
> This doesn't seem right because returning 1 here causes pci_walk_bus()
> to terminate, which means we won't set dev->error_state or notify
> drivers for any devices we haven't visited yet.
> 
> > +	}
> >  	if (!dev->driver ||
> >  		!dev->driver->err_handler ||
> >  		!dev->driver->err_handler->error_detected) {
> 
> If none of the drivers in the affected hierarchy supports error handling,
> I think the call tree looks like this:
> 
>     do_recovery                                 # uncorrectable only
>         broadcast_error_message(dev, ..., report_error_detected)
>             result_data.result = CAN_RECOVER
>             pci_walk_bus(..., report_error_detected)
>                 report_error_detected           # (each dev in subtree)
>                     return 0                    # no change to result
>             return result_data.result
>         broadcast_error_message(dev, ..., report_mmio_enabled)
>             result_data.result = PCI_ERS_RESULT_RECOVERED
>             pci_walk_bus(..., report_mmio_enabled)
>                 report_mmio_enabled             # (each dev in subtree)
>                     return 0                    # no change to result
>         dev_info("recovery successful")
> 
> Specifically, there are no error_handler functions, so we never change
> result_data.result, and the default is that we treat the error as
> "recovered successfully."  That seems broken.  An uncorrectable error
> is by definition recoverable only by device-specific software, i.e.,
> the driver.  We didn't call any drivers, so we can't have recovered
> anything.
> 
> What do you think of the following alternative?  I don't know why you
> checked for bridge devices in your patch, so I don't know whether
> that's important here or not.
> 
> diff --git a/drivers/pci/pcie/aer/aerdrv_core.c
> b/drivers/pci/pcie/aer/aerdrv_core.c
> index 06bad96..a109c68 100644
> --- a/drivers/pci/pcie/aer/aerdrv_core.c
> +++ b/drivers/pci/pcie/aer/aerdrv_core.c
> @@ -231,11 +231,11 @@ static int report_error_detected(struct pci_dev *dev,
> void *data)
>  				   dev->driver ?
>  				   "no AER-aware driver" : "no driver");
>  		}
> -		return 0;
> +		vote = PCI_ERS_RESULT_DISCONNECT;
> +	} else {
> +		err_handler = dev->driver->err_handler;
> +		vote = err_handler->error_detected(dev, result_data->state);
>  	}
> -
> -	err_handler = dev->driver->err_handler;
> -	vote = err_handler->error_detected(dev, result_data->state);
>  	result_data->result = merge_result(result_data->result, vote);
>  	return 0;
>  }

This would definitely set the error_state for all devices correctly. However, with the 
current implementation of merge_result(), won't we still end up reporting successful 
recovery ? The following case statement in merge_result() can set back the result 
from PCI_ERS_RESULT_DISCONNECT to PCI_ERS_RESULT_NEED_RESET for a subsequent device 
on the bus which returned PCI_ERS_RESULT_NEED_RESET from its error_detected callback . 

merge_result()
...
        case PCI_ERS_RESULT_DISCONNECT:
                if (new == PCI_ERS_RESULT_NEED_RESET)
                        orig = new;
                break;

This would mean do_recovery() proceeds along to the next broadcast_message and 
ultimately report success. Right ? Let me know if I am missing something.

I looked at a few options and the following looked more promising. Thoughts ?

Introduce a new pci_ers_result enum PCI_ERS_RESULT_NO_AER_DRIVER and make changes as follows.

diff --git a/drivers/pci/pcie/aer/aerdrv.h b/drivers/pci/pcie/aer/aerdrv.h
index 94a7598..149ba79 100644
--- a/drivers/pci/pcie/aer/aerdrv.h
+++ b/drivers/pci/pcie/aer/aerdrv.h
@@ -87,6 +87,9 @@ struct aer_broadcast_data {
 static inline pci_ers_result_t merge_result(enum pci_ers_result orig,
 		enum pci_ers_result new)
 {
+	if (new == PCI_ERS_RESULT_NO_AER_DRIVER)
+		return new;
+
 	if (new == PCI_ERS_RESULT_NONE)
 		return orig;
 
diff --git a/drivers/pci/pcie/aer/aerdrv_core.c b/drivers/pci/pcie/aer/aerdrv_core.c
index 06bad96..729580a 100644
--- a/drivers/pci/pcie/aer/aerdrv_core.c
+++ b/drivers/pci/pcie/aer/aerdrv_core.c
@@ -231,11 +231,12 @@ static int report_error_detected(struct pci_dev *dev, void *data)
 				   dev->driver ?
 				   "no AER-aware driver" : "no driver");
 		}
-		return 0;
+		vote = PCI_ERS_RESULT_NO_AER_DRIVER;
+	} else {
+		err_handler = dev->driver->err_handler;
+		vote = err_handler->error_detected(dev, result_data->state);
 	}
 
-	err_handler = dev->driver->err_handler;
-	vote = err_handler->error_detected(dev, result_data->state);
 	result_data->result = merge_result(result_data->result, vote);
 	return 0;
 }
diff --git a/include/linux/pci.h b/include/linux/pci.h
index ee21795..fb7e869 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -538,6 +538,9 @@ enum pci_ers_result {
 
 	/* Device driver is fully recovered and operational */
 	PCI_ERS_RESULT_RECOVERED = (__force pci_ers_result_t) 5,
+
+	/* No AER capabilities registered for the driver */
+	PCI_ERS_RESULT_NO_AER_DRIVER = (__force pci_ers_result_t) 6,
 };
 
 /* PCI bus error event callbacks */

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