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SubjectRe: [PATCH] vfio/pci: Support error recovery
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On 12/01/2016 12:04 PM, Alex Williamson wrote:
> On Sun, 27 Nov 2016 19:34:17 +0800
> Cao jin <caoj.fnst@cn.fujitsu.com> 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@redhat.com
>> CC: mst@redhat.com
>> Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
>> ---
>> 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;
>
>
>
> .
>




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