Re: [Xen-devel] [RFC Patch] xen/pt: Emulate FLR capability

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Chao Gao <chao.gao@xxxxxxxxx>
> Sent: 06 September 2019 10:01
> To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; qemu-devel@xxxxxxxxxx; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Anthony Perard <anthony.perard@xxxxxxxxxx>; Paul 
> Durrant
> <Paul.Durrant@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>
> Subject: Re: [RFC Patch] xen/pt: Emulate FLR capability
> 
> On Thu, Aug 29, 2019 at 12:21:11PM +0200, Roger Pau Monné wrote:
> >On Thu, Aug 29, 2019 at 05:02:27PM +0800, Chao Gao wrote:
> >> Currently, for a HVM on Xen, no reset method is virtualized. So in a VM's
> >> perspective, assigned devices cannot be reset. But some devices rely on PCI
> >> reset to recover from hardware hangs. When being assigned to a VM, those
> >> devices cannot be reset and won't work any longer if a hardware hang 
> >> occurs.
> >> We have to reboot VM to trigger PCI reset on host to recover the device.
> >>
> >> This patch exposes FLR capability to VMs if the assigned device can be 
> >> reset on
> >> host. When VM initiates an FLR to a device, qemu cleans up the device 
> >> state,
> >> (including disabling of intx and/or MSI and unmapping BARs from guest, 
> >> deleting
> >> emulated registers), then initiate PCI reset through 'reset' knob under the
> >> device's sysfs, finally initialize the device again.
> >
> >I think you likely need to deassign the device from the VM, perform
> >the reset, and then assign the device again, so that there's no Xen
> >internal state carried over prior to the reset?
> 
> Yes. It is the safest way. But here I want to present the feature as FLR
> (such that the device driver in guest can issue PCI reset whenever
> needed and no change is needed to device driver).  Current device
> deassignment notifies guest that the device is going to be removed
> It is not the standard PCI reset. Is it possible to make guest unaware
> of the device deassignment to emulate a standard PCI reset?

It should be, I would have thought. QEMU emulates all config space so any 
config access by the guest would be unaffected by de-assignment. The BARs and 
interrupts would be unmapped... but that's what you'd want anyway.

> In my mind,
> we can expose do_pci_remove/add to qemu or rewrite them in qemu (but
> don't remove the device from guest's PCI hierarchy). Do you think it is
> the right direction?

Long term I think we want to get pass-through emulation out of QEMU and into 
Xen.

  Paul

> 
> Thanks
> Chao

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