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

[Huang Rui <ray.huang@amd.com>]

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?

Thanks,
Ray

> > And yes, it's temporary workaround, we should figure out
> > the root cause.
> 
> Right, that's the only way to approach this, imo.
> 
> Jan