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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Mar 21, 2023 at 07:49:26PM +0800, Huang Rui wrote:
> On Tue, Mar 21, 2023 at 06:20:03PM +0800, Jan Beulich wrote:
> > On 21.03.2023 11:14, Huang Rui wrote:
> > > On Tue, Mar 21, 2023 at 05:41:57PM +0800, Jan Beulich wrote:
> > >> On 21.03.2023 10:36, Huang Rui wrote:
> > >>> On Wed, Mar 15, 2023 at 12:02:35AM +0800, Jan Beulich wrote:
> > >>>> On 12.03.2023 08:54, Huang Rui wrote:
> > >>>>> --- a/xen/drivers/vpci/header.c
> > >>>>> +++ b/xen/drivers/vpci/header.c
> > >>>>> @@ -392,7 +392,7 @@ static void cf_check bar_write(
> > >>>>>       * Xen only cares whether the BAR is mapped into the p2m, so 
> > >>>>> allow BAR
> > >>>>>       * writes as long as the BAR is not mapped into the p2m.
> > >>>>>       */
> > >>>>> -    if ( bar->enabled )
> > >>>>> +    if ( pci_conf_read16(pdev->sbdf, PCI_COMMAND) & 
> > >>>>> PCI_COMMAND_MEMORY )
> > >>>>>      {
> > >>>>>          /* If the value written is the current one avoid printing a 
> > >>>>> warning. */
> > >>>>>          if ( val != (uint32_t)(bar->addr >> (hi ? 32 : 0)) )
> > >>>>
> > >>>> ... bar->enabled doesn't properly reflect the necessary state? It
> > >>>> generally shouldn't be necessary to look at the physical device's
> > >>>> state here.
> > >>>>
> > >>>> Furthermore when you make a change in a case like this, the
> > >>>> accompanying comment also needs updating (which might have clarified
> > >>>> what, if anything, has been wrong).
> > >>>>
> > >>>
> > >>> That is the problem that we start domU at the first time, the enable 
> > >>> flag
> > >>> will be set while the passthrough device would like to write the real 
> > >>> pcie
> > >>> bar on the host.
> > >>
> > >> A pass-through device (i.e. one already owned by a DomU) should never
> > >> be allowed to write to the real BAR. But it's not clear whether I'm not
> > >> misinterpreting what you said ...
> > >>
> > > 
> > > OK. Thanks to clarify this. May I know how does a passthrough device 
> > > modify
> > > pci bar with correct behavior on Xen?
> > 
> > A pass-through device may write to the virtual BAR, changing where in its
> > own memory space the MMIO range appears. But it cannot (and may not) alter
> > where in host memory space the (physical) MMIO range appears.
> > 
> 
> Thanks, but we found if dom0 is PV domain, the passthrough device will
> access this function to write the real bar.

I'm very confused now, are you trying to use vPCI with HVM domains?

As I understood it you are attempting to enable PCI passthrough for
HVM guests from a PVH dom0, but now you say your dom0 is PV?

Thanks, Roger.