Re: [Qemu-devel] [Qemu-ppc] [PATCH 2/3] VFIO: Clear INTx pending state on EEH reset

[Gavin Shan <gwshan@linux.vnet.ibm.com>]

On Mon, Mar 16, 2015 at 03:05:32PM +1100, Benjamin Herrenschmidt wrote:
>On Mon, 2015-03-16 at 12:04 +1100, Gavin Shan wrote:
>> 
>> 
>> (2) QEMU sends IOCTL commands to host to disable MSIx and enable INTx. At
>> this stage the INTx is still masked. At later point, the guest is requesting
>> unmasking INTx, which is captured by host. Host checks and founds pending
>> INTx, which is sent to QEMU. In QEMU INTx handler (vfio_intx_interrupt()),
>> the mmap'ed regions are disabled, "intx.pending" is set and a timer is started
>> to reenable mmap'ed regions if "intx.pending" is cleared there. However,
>> "intx.pending" is only cleared upon BAR access in slow path, which is never
>> happing.
>> 
>> (3) After guest disables MSIx and issue EEH reset, the device driver starts
>> to check its firmware state by reading MMIO register, which isn't completed
>> by QEMU VFIO BAR slow path (Note: fast path supported by mmaped regions have
>> been disabled). Eventually, the guest hangs on reading MMIO register. With
>> this patch applied to QEMU, I didn't see the problem again. 
>
>Note that it might be a good idea to disable INTx (and synchronize with a cfg
>read of some sort) around resetting a device.
>
>Otherwise, you may hit a known issue if the device is behind a switch and has
>sent the INTx "assert" message, and not the "deassert" one before it gets reset.
>
>That can cause the INTx to effectively be "stuck" in the switch preventing a
>subsequent one from being delivered.
>

Yeah, It makes more sense to disable INTx before issuing EEH reset. I verified
that disabling INTx interrupt upon EEH reset can avoid the issue as well. I'll
post updated patch accordingly if Alex Williamson doesn't object.

Thanks,
Gavin

>Cheers,
>Ben.
>
>