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

Bjorn Helgaas
2025-01-10T22:25:27Z

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.

> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
> 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
>