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Re: [Qemu-devel] [patch v5 11/12] vfio: device may stuck in D3 when doin

From: Alex Williamson
Subject: Re: [Qemu-devel] [patch v5 11/12] vfio: device may stuck in D3 when doing aer recovery
Date: Thu, 31 Mar 2016 09:44:04 -0600

On Thu, 31 Mar 2016 14:55:07 +0800
Chen Fan <address@hidden> wrote:

> On 03/25/2016 10:22 AM, Alex Williamson wrote:
> > On Fri, 25 Mar 2016 09:38:09 +0800
> > Chen Fan <address@hidden> wrote:
> >  
> >> On 03/25/2016 06:54 AM, Alex Williamson wrote:  
> >>> On Wed, 23 Mar 2016 18:12:06 +0800
> >>> Cao jin <address@hidden> wrote:
> >>>     
> >>>> From: Chen Fan <address@hidden>
> >>>>
> >>>> when a physical device aer occurred, the device state probably
> >>>> is not in D0 in a short time, if we recover the device quickly.
> >>>> we may stuck in D3 state when force to change device state to D0.
> >>>> we may need to wait for a short time to inject the error to guest.
> >>>>
> >>>> Signed-off-by: Chen Fan <address@hidden>
> >>>> ---
> >>>>    hw/vfio/pci.c | 3 +++
> >>>>    1 file changed, 3 insertions(+)
> >>>>
> >>>> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> >>>> index 25fc095..5216e7f 100644
> >>>> --- a/hw/vfio/pci.c
> >>>> +++ b/hw/vfio/pci.c
> >>>> @@ -2658,6 +2658,9 @@ static void vfio_err_notifier_handler(void *opaque)
> >>>>            msg.severity = isfatal ? PCI_ERR_ROOT_CMD_FATAL_EN :
> >>>>                                     PCI_ERR_ROOT_CMD_NONFATAL_EN;
> >>>>    
> >>>> +        /* wait a bit to ensure aer device is ready */
> >>>> +        usleep(2 * 1000);  
> >>> Where does this number come from?  Why would the device be in D3?  I
> >>> don't understand this at all.  
> >> Hi Alex,
> >>
> >>       when I tested the code in my environment, I found that when I used
> >> the aer-inject module to inject a fake aer error to device on host, the 
> >> qemu
> >> would throw out the message "vfio: Unable to power on device, stuck in D3"
> >> on and off. if I use "gdb" to debug the vfio_pci_pre_reset, the phenomenon
> >> would not appearance, I just thought it should be some timing race issue,
> >> so I use a sleep() to wait 2ms (double the reset time of 1ms) to ensure the
> >> device state is ready. maybe the root reason still need to be
> >> investigated deeply.  
> > Yes, it sounds like you need to investigate this further, the delay is
> > arbitrary and perhaps suggests a race that needs to be fixed
> > correctly.  Thanks,  
> Hi Alex,
>      after done some investigation of the problem, I found that only 
> when the injected
>   error is fatal, the problem will appear. because in aer do_recovery, 
> host will call reset_link
> on the root port, which would invoke pci_reset_bridge_secondary_bus in 
> aer_root_reset,
> that would reset the bridge and all the device under that. so when qemu 
> receive the aer
> notification, then propagate the error to guest, guest does the same way 
> to perform the
> recovery, if the guest `reset_link` that will call the vfio_pre_reset 
> done at the stage of host
> bridge reset, the device status would probable stick in D3.
> so I think after qemu receive the aer notification, we should wait for 
> enough time to
> ensure the bridge has been reset completely. I just use sleep <=10ms to 
> test the code,
> seems still appear the message "vfio: Unable to power on device, stuck 
> in D3". so I think
> we should sleep 100ms to ensure the delay sufficient. I have tested that 
> code 100+ times
> by inject aer error. the issue no longer appears.

I'm not satisfied with this.  pci_reset_bridge_secondary_bus() is
invoked by both the host AER code and the guest AER code, the latter
via the vfio PCI hot reset interface.  The
pci_reset_bridge_secondary_bus() function includes the spec defined
delay by which point all the devices should be operational again.  The
spec also defines that devices are in D0 after reset, which implies
that the only reason we would ever be seeing a device in D3 is if we're
reading the device while it is still in reset or before it has
recovered from reset.  That implies that either
pci_reset_bridge_secondary_bus() is not waiting long enough or QEMU is
allowing one device to call vfio_pre_reset while another device is
still in reset.  I suspect QEMU serializes reset such that the latter
case is not possible, which means that you might have a device that
takes longer to reset than the spec defines.  Such a quirk should be
handled in the host kernel reset, not by adding arbitrary delays in
userspace code.  Thanks,


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