Re: [RFC XEN PATCH 2/6] vpci: accept BAR writes if dom0 is PVH

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Mar 22, 2023 at 01:48:30PM +0100, Jan Beulich wrote:
> On 22.03.2023 13:33, Huang Rui wrote:
> > I traced that while we do pci-assignable-add, we will follow below trace to
> > bind the passthrough device.
> > 
> > pciassignable_add()->libxl_device_pci_assignable_add()->libxl__device_pci_assignable_add()->pciback_dev_assign()
> > 
> > Then kernel xen-pciback driver want to add virtual configuration spaces. In
> > this phase, the bar_write() in xen hypervisor will be called. I still need
> > a bit more time to figure the exact reason. May I know where the
> > xen-pciback driver would trigger a hvm_io_intercept to xen hypervisor?
> 
> Any config space access would. And I might guess ...
> 
> > [  309.719049] xen_pciback: wants to seize 0000:03:00.0
> > [  462.911251] pciback 0000:03:00.0: xen_pciback: probing...
> > [  462.911256] pciback 0000:03:00.0: xen_pciback: seizing device
> > [  462.911257] pciback 0000:03:00.0: xen_pciback: pcistub_device_alloc
> > [  462.911261] pciback 0000:03:00.0: xen_pciback: initializing...
> > [  462.911263] pciback 0000:03:00.0: xen_pciback: initializing config
> > [  462.911265] pciback 0000:03:00.0: xen-pciback: initializing virtual configuration space
> > [  462.911268] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x00
> > [  462.911271] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x02
> > [  462.911284] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x04
> > [  462.911286] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x3c
> > [  462.911289] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x3d
> > [  462.911291] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x0c
> > [  462.911294] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x0d
> > [  462.911296] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x0f
> > [  462.911301] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x10
> > [  462.911306] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x14
> > [  462.911309] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x18
> > [  462.911313] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x1c
> > [  462.911317] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x20
> > [  462.911321] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x24
> > [  462.911325] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x30
> > [  462.911358] pciback 0000:03:00.0: Found capability 0x1 at 0x50
> > [  462.911361] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x50
> > [  462.911363] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x52
> > [  462.911368] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x54
> > [  462.911371] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x56
> > [  462.911373] pciback 0000:03:00.0: xen-pciback: added config field at offset 0x57
> > [  462.911386] pciback 0000:03:00.0: Found capability 0x5 at 0xa0
> > [  462.911388] pciback 0000:03:00.0: xen-pciback: added config field at offset 0xa0
> > [  462.911391] pciback 0000:03:00.0: xen-pciback: added config field at offset 0xa2
> > [  462.911405] pciback 0000:03:00.0: xen_pciback: enabling device
> > [  462.911412] pciback 0000:03:00.0: enabling device (0006 -> 0007)
> > [  462.911658] Already setup the GSI :28
> > [  462.911668] Already map the GSI :28 and IRQ: 115
> > [  462.911684] pciback 0000:03:00.0: xen_pciback: save state of device
> > [  462.912154] pciback 0000:03:00.0: xen_pciback: resetting (FLR, D3, etc) the device
> > [  463.954998] pciback 0000:03:00.0: xen_pciback: reset device
> 
> ... it is actually the reset here, saving and then restoring config space.
> If e.g. that restore was done "blindly" (i.e. simply writing fields low to
> high), then memory decode would be re-enabled before the BARs are written.

The problem is also that we don't tell vPCI that the device has been
reset, so the current cached state in pdev->vpci is all out of date
with the real device state.

I didn't hit this on my test because the device I was using had no
reset support.

I don't think it's feasible for Xen to detect all the possible reset
methods dom0 might use, as some of those are device specific for
example.

We would have to introduce a new hypercall that clears all vPCI device
state, PHYSDEVOP_pci_device_reset for example.  This will involve
adding proper cleanup functions, as the current code in
vpci_remove_device() only deals with allocated memory (because so far
devices where not deassigned) but we now also need to make sure
MSI(-X) interrupts are torn down and freed, and will also require
removing any mappings of BARs into the dom0 physmap.

Thanks, Roger.