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

[Huang Rui <ray.huang@xxxxxxx>]

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.

Thanks,
Ray