Re: [PATCH] vfio/pci: Support error recovery

[Cao jin]

On 12/01/2016 12:04 PM, Alex Williamson wrote:
> On Sun, 27 Nov 2016 19:34:17 +0800
> Cao jin <caoj.fnst@xxxxxxxxxxxxxx> wrote:
> 
>> It is user space driver's or device-specific driver's(in guest) responsbility
>> to do a serious recovery when error happened. Link-reset is one part of
>> recovery, when pci device is assigned to VM via vfio, link-reset will do
>> twice in host & guest separately, which will cause many trouble for a
>> successful recovery, so, disable the vfio-pci's link-reset in aer driver
>> in host, this is a keypoint for guest to do error recovery successfully.
>>
>> CC: alex.williamson@xxxxxxxxxx
>> CC: mst@xxxxxxxxxx
>> Signed-off-by: Cao jin <caoj.fnst@xxxxxxxxxxxxxx>
>> ---
>> This is actually a RFC version(has debug lines left), and has minor changes in
>> aer driver, so I think maybe it is better not to CC pci guys in this round.
>> Later will do.
>>
>>  drivers/pci/pcie/aer/aerdrv_core.c  | 12 ++++++-
>>  drivers/vfio/pci/vfio_pci.c         | 63 +++++++++++++++++++++++++++++++++++--
>>  drivers/vfio/pci/vfio_pci_private.h |  2 ++
>>  3 files changed, 74 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/pci/pcie/aer/aerdrv_core.c b/drivers/pci/pcie/aer/aerdrv_core.c
>> index 521e39c..289fb8e 100644
>> --- a/drivers/pci/pcie/aer/aerdrv_core.c
>> +++ b/drivers/pci/pcie/aer/aerdrv_core.c
>> @@ -496,7 +496,17 @@ static void do_recovery(struct pci_dev *dev, int severity)
>>  			"error_detected",
>>  			report_error_detected);
>>  
>> -	if (severity == AER_FATAL) {
>> +	/* vfio-pci as a general meta driver, it actually couldn't do any real
>> +	 * recovery for device. It is user space driver, or device-specific
>> +	 * driver in guest who should take care of the serious error recovery,
>> +	 * link reset actually is one part of whole recovery. Doing reset_link
>> +	 * in aer driver of host kernel for vfio-pci devices will cause many
>> +	 * trouble for user space driver or guest's device-specific driver,
>> +	 * for example: the serious recovery often need to read register in
>> +	 * config space, but if register reading happens during link-resetting,
>> +	 * it is quite possible to return invalid value like all F's, which
>> +	 * will result in unpredictable error. */
>> +	if (severity == AER_FATAL && strcmp(dev->driver->name, "vfio-pci")) {
>>  		result = reset_link(dev);
>>  		if (result != PCI_ERS_RESULT_RECOVERED)
>>  			goto failed;
> 
> This is not acceptable.  If we want to make a path through AER recovery
> that does not reset the link, there should be a way for the driver to
> request it.  Testing the driver name is a horrible hack.  The other
> question is what guarantees does vfio make that the device does get
> reset?  

I am not sure how vfio guarantee that...When device is assigned to VM,
we have that guarantees(aer driver in guest driver will do that), so I
think it is a well-behaved user space driver's responsibility to do link
reset?  And I think if there is a user space driver, it is surely its
responsibility to consider how to do serious error recovery, like I said
before, vfio, as a general meta driver, it surely don't know how each
device does its specific recovery, except bus/slot reset

> If an AER fault occurs and the user doesn't do a reset, what
> happens when that device is released and a host driver tries to make
> use of it?  The user makes no commitment to do a reset and there are
> only limited configurations where we even allow the user to perform a
> reset.
> 

Limited? Do you mean the things __pci_dev_reset() can do?

...
> 
>> +	aer_cap_offset = pci_find_ext_capability(vdev->pdev, PCI_EXT_CAP_ID_ERR);
>> +	ret = pci_read_config_dword(vdev->pdev, aer_cap_offset + 
>> +                                    PCI_ERR_UNCOR_STATUS, &uncor_status);
>> +        if (ret)
>> +                return PCI_ERS_RESULT_DISCONNECT;
>> +
>> +	pr_err("device %d got AER detect notification. uncorrectable error status = 0x%x\n", pdev->devfn, uncor_status);//to be removed
>>  	mutex_lock(&vdev->igate);
>> +    
>> +	vdev->aer_recovering = true;
>> +	reinit_completion(&vdev->aer_error_completion);
>> +
>> +	/* suspend config space access from user space,
>> +	 * when vfio-pci's error recovery process is on */
>> +	pci_cfg_access_trylock(vdev->pdev);
>>  
>> -	if (vdev->err_trigger)
>> -		eventfd_signal(vdev->err_trigger, 1);
>> +	if (vdev->err_trigger && uncor_status) {
>> +		pr_err("device %d signal uncor status to user space", pdev->devfn);//may be removed
>> +		/* signal uncorrectable error status to user space */
>> +		eventfd_signal(vdev->err_trigger, uncor_status);
>> +        }
> 
> } else... what?  By bypassing the AER link reset we've assumed
> responsibility for resetting the device.  Even if we signal the user,
> what guarantee do we have that the device is recovered?
> 

else...consider it as a fake error notification and ignore?

I am not sure I understand your thoughts totally, but it seems my
previous comments apply, that is: it is well-behaved user space driver's
responsibility to do a serious recovery.

In my understanding, user space driver has 2 category: one is VM(has
guest OS running inside), the other is ordinary user space program.

When device is assigned to a VM, (qemu + guest OS) will do fully steps
to do recovery(roughly is what struct pci_error_handlers has). So,
equally, if it is a ordinary user space program acting as the driver,
the responsibility belongs to it.


Sincerely,
Cao jin
>>  
>>  	mutex_unlock(&vdev->igate);
>>  
>> @@ -1199,8 +1232,34 @@ static pci_ers_result_t vfio_pci_aer_err_detected(struct pci_dev *pdev,
>>  	return PCI_ERS_RESULT_CAN_RECOVER;
>>  }
>>  
>> +static void vfio_pci_aer_resume(struct pci_dev *pdev)
>> +{
>> +	struct vfio_pci_device *vdev;
>> +	struct vfio_device *device;
>> +
>> +	device = vfio_device_get_from_dev(&pdev->dev);
>> +	if (device == NULL)
>> +		return;
>> +
>> +	vdev = vfio_device_data(device);
>> +	if (vdev == NULL) {
>> +		vfio_device_put(device);
>> +		return;
>> +	}
>> +
>> +	/* vfio-pci's error recovery is done, time to
>> +	 * resume pci config space's accesses */
>> +	pci_cfg_access_unlock(vdev->pdev);
>> +
>> +	vdev->aer_recovering = false;
>> +	complete_all(&vdev->aer_error_completion);
>> +
>> +	vfio_device_put(device);
>> +}
>> +
>>  static const struct pci_error_handlers vfio_err_handlers = {
>>  	.error_detected = vfio_pci_aer_err_detected,
>> +	.resume         = vfio_pci_aer_resume,
>>  };
>>  
>>  static struct pci_driver vfio_pci_driver = {
>> diff --git a/drivers/vfio/pci/vfio_pci_private.h b/drivers/vfio/pci/vfio_pci_private.h
>> index 8a7d546..ebf1041 100644
>> --- a/drivers/vfio/pci/vfio_pci_private.h
>> +++ b/drivers/vfio/pci/vfio_pci_private.h
>> @@ -83,6 +83,8 @@ struct vfio_pci_device {
>>  	bool			bardirty;
>>  	bool			has_vga;
>>  	bool			needs_reset;
>> +	bool			aer_recovering;
>> +	struct completion	aer_error_completion;
>>  	struct pci_saved_state	*pci_saved_state;
>>  	int			refcnt;
>>  	struct eventfd_ctx	*err_trigger;
> 
> 
> 
> .
>