Re: [PATCH 2/3] vmd: disable MSI remapping bypass under Xen

[Jonathan Derrick <jonathan.derrick@xxxxxxxxx>]

Hi Bjorn,

On 1/10/25 3:25 PM, Bjorn Helgaas wrote:
> Match historical subject line style for prefix and capitalization:
>
>    PCI: vmd: Set devices to D0 before enabling PM L1 Substates
>    PCI: vmd: Add DID 8086:B06F and 8086:B60B for Intel client SKUs
>    PCI: vmd: Fix indentation issue in vmd_shutdown()
>
> On Fri, Jan 10, 2025 at 03:01:49PM +0100, Roger Pau Monne wrote:
> > MSI remapping bypass (directly configuring MSI entries for devices on the VMD
> > bus) won't work under Xen, as Xen is not aware of devices in such bus, and
> > hence cannot configure the entries using the pIRQ interface in the PV case, and
> > in the PVH case traps won't be setup for MSI entries for such devices.
> >
> > Until Xen is aware of devices in the VMD bus prevent the
> > VMD_FEAT_CAN_BYPASS_MSI_REMAP capability from being used when running as any
> > kind of Xen guest.
> Wrap to fit in 75 columns.
>
> Can you include a hint about *why* Xen is not aware of devices below
> VMD?  That will help to know whether it's a permanent unfixable
> situation or something that could be done eventually.
I wasn't aware of the Xen issue with VMD but if I had to guess it's 
probably due to the special handling of the downstream device into the 
dmar table.
[1] 4fda230ecddc2
[2] 9b4a824b889e1


> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> >   drivers/pci/controller/vmd.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
> > index 264a180403a0..d9b7510ace29 100644
> > --- a/drivers/pci/controller/vmd.c
> > +++ b/drivers/pci/controller/vmd.c
> > @@ -965,6 +965,15 @@ static int vmd_probe(struct pci_dev *dev, const struct 
> > pci_device_id *id)
> >         struct vmd_dev *vmd;
> >         int err;
> >   
> > +	if (xen_domain())
> > +               /*
> > +                * Xen doesn't have knowledge about devices in the VMD bus.
> Also here.
>
> > +                * Bypass of MSI remapping won't work in that case as direct
> > +                * write to the MSI entries won't result in functional
> > +                * interrupts.
> > +                */
> > +               features &= ~VMD_FEAT_CAN_BYPASS_MSI_REMAP;
> > +
> >         if (resource_size(&dev->resource[VMD_CFGBAR]) < (1 << 20))
> >                 return -ENOMEM;
> >   
> > --
> > 2.46.0