linux-kernel - Re: [PATCH] vfio/pci: Support error recovery

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Date:   Mon, 28 Nov 2016 17:32:15 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <izumi.taku@...fujitsu.com>, <alex.williamson@...hat.com>
Subject: Re: [PATCH] vfio/pci: Support error recovery



On 11/28/2016 11:00 AM, Michael S. Tsirkin wrote:
> On Sun, Nov 27, 2016 at 07:34:17PM +0800, Cao jin 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@...hat.com
>> CC: mst@...hat.com
>> Signed-off-by: Cao jin <caoj.fnst@...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. */
>
> Fix multi-comment style please.
>
>> +	if (severity == AER_FATAL && strcmp(dev->driver->name, "vfio-pci")) {
>
> You really want some flag in the device, or something similar.
> Also, how do we know driver is not going away at this point?
>

I didn't think of this condition, and I don't quite follow how would 
driver go away?(device has error happened, then is removed?)

>>   		result = reset_link(dev);
>>   		if (result != PCI_ERS_RESULT_RECOVERED)
>>   			goto failed;

>> @@ -1187,10 +1200,30 @@ static pci_ers_result_t vfio_pci_aer_err_detected(struct pci_dev *pdev,
>>   		return PCI_ERS_RESULT_DISCONNECT;
>>   	}
>>
>> +	/* get device's uncorrectable error status as soon as possible,
>> +	 * and signal it to user space. The later we read it, the possibility
>> +	 * the register value is mangled grows. */
>> +	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 */
>
> what about access to memory etc? Do you need to suspend this as well?
>

Yes, this question came into my mind a little bit, but I didn't see some 
existing APIs like pci_cfg_access_xxx which can help to do this.(I am 
still not familiar with kernel)

>> +	pci_cfg_access_trylock(vdev->pdev);
>
> If you trylock, you need to handle failure.

try lock returns 0 if access is already locked, 1 otherwise. Is it 
necessary to check its return value?


-- 
Sincerely,
Cao jin