Re: [PATCH v2 2/3] vmd: disable MSI remapping bypass under Xen

[Bjorn Helgaas <helgaas@kernel.org>]

The subject line convention for this file is:

  PCI: vmd: Disable MSI remapping ...

On Tue, Jan 14, 2025 at 11:33:12AM +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.
> 
> The MSI remapping bypass is an optional feature of VMD bridges, and hence
> when running under Xen it will be masked and devices will be forced to
> redirect its interrupts from the VMD bridge.  That mode of operation must
> always be supported by VMD bridges and works when Xen is not aware of
> devices behind the VMD bridge.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
> Changes since v1:
>  - Add xen header.
>  - Expand comment.
> ---
>  drivers/pci/controller/vmd.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
> index 264a180403a0..33c9514bd926 100644
> --- a/drivers/pci/controller/vmd.c
> +++ b/drivers/pci/controller/vmd.c
> @@ -17,6 +17,8 @@
>  #include <linux/rculist.h>
>  #include <linux/rcupdate.h>
>  
> +#include <xen/xen.h>
> +
>  #include <asm/irqdomain.h>
>  
>  #define VMD_CFGBAR	0
> @@ -965,6 +967,23 @@ 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
> +		 * because the config space of devices behind the VMD bridge is
> +		 * not known to Xen, and hence Xen cannot discover or configure
> +		 * them in any way.
> +		 *
> +		 * Bypass of MSI remapping won't work in that case as direct
> +		 * write by Linux to the MSI entries won't result in functional
> +		 * interrupts, as it's Xen the entity that manages the host

"... as Xen is the entity that ..." ?

> +		 * interrupt controller and must configure interrupts.
> +		 * However multiplexing of interrupts by the VMD bridge will
> +		 * work under Xen, so force the usage of that mode which must
> +		 * always be supported by VMD bridges.
> +		 */
> +		features &= ~VMD_FEAT_CAN_BYPASS_MSI_REMAP;

Since the comment is so long, I would add braces even though it's only
a single statement.  Or maybe moving the comment above the "if" would
make more sense.

>  	if (resource_size(&dev->resource[VMD_CFGBAR]) < (1 << 20))
>  		return -ENOMEM;
>  
> -- 
> 2.46.0
>