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

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Date:   Tue, 6 Dec 2016 05:46:15 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Cao jin <caoj.fnst@...fujitsu.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        izumi.taku@...fujitsu.com
Subject: Re: [PATCH] vfio/pci: Support error recovery

On Thu, Dec 01, 2016 at 09:40:23PM +0800, Cao jin wrote:
> 
> 
> On 12/01/2016 12:51 PM, Michael S. Tsirkin wrote:
> > On Wed, Nov 30, 2016 at 09:04:13PM -0700, Alex Williamson wrote:
> >> On Sun, 27 Nov 2016 19:34:17 +0800
> >> Cao jin <caoj.fnst@...fujitsu.com> wrote:
> >>
> 
> >>> @@ -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. */
> >>
> >> Bad comment style (throughout).
> >>
> >>> +	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);
> > 
> > What does this mean that we trigger with uncor_status as opposed to 1?
> > 
> 
> I guess you missed the changelog in qemu patchset's cover letter, see if
> it helps(copy from cover letter):
> 
> 5. Change what eventfd signals(check vfio driver patch). Before,
> vfio-pci driver add 1 to the counter, which doesn't have meaning, just
> for notification. Now, vfio-pci's error detected handler read the
> uncorrectable error status and signal it to qemu.

I don't think you can use an eventfd to send values like this.

eventfd does a sum of these values, so sending e.g.
value 2 will look the same as sending value 1 twice.



> Why? When testing previous version(host aer driver still reset link),
> found that there is quite possibility that register reading returns the
> invalid value 0xFFFFFFFF, which results in all(2 in my environment)
> qemu's vfio-pci function send AER to root port while I only inject error
> into one function. This is unreasonable, and I think it is the result of
> reading during device reset.
>
> Previous patch does considered to find the
> real function who has error happened, but won't work under the situation
> I found. So move the register reading as early as possible would be the
> nature thoughts, and it is moved into vfio-pci driver. Although now
> reset_link is disabled in aer driver, get the uncor error status as
> early as possible still make sense, for example: if user space driver
> does device reset once receiving the error notification, and then read
> register.

I had trouble understanding the above. Let me ask you this:
should we try to avoid triggering uncorrectable errors?
Aren't any such errors likely to crash guests?

> -- 
> Sincerely,
> Cao jin
>