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

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Message-ID: <20250110222525.GA318386@bhelgaas>
Date: Fri, 10 Jan 2025 16:25:25 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Roger Pau Monne <roger.pau@...rix.com>
Cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	linux-pci@...r.kernel.org,
	Nirmal Patel <nirmal.patel@...ux.intel.com>,
	Jonathan Derrick <jonathan.derrick@...ux.dev>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 2/3] vmd: disable MSI remapping bypass under Xen

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@...rix.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
>