Re: [PATCH v7] PCI: vmd: Add feature to scan BIOS enumerated devices.

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Jun 29, 2026 at 04:50:25PM +0000, Nirmal Patel wrote:
> Newer VMD  with device ID 0x28c1 has unique settings compared to its
> predecessor where BIOS enumerates the entire VMD device tree and
> assigns respective configurations.
> 
> VMD configuration BAR0 carries over from GNR legacy VMD as the mechanism
> to access the configuration space of the devices owned by VMD. The size
> of this window is fixed at 256 MB, where each function consumes 4 KB and
> every bus consumes 1 MB.
> 
> The shadow and scratchpad registers have been relocated from the VMD
> configuration space to the VMD MMIO space in VMD BAR4/BAR5, otherwise
> refers to as MEMBAR2 or MSI-X bar.
> 
> VMD MSI-X remapping enable/disable is no longer supported.
> 
> All the VMD driver code needs to do is to obtain bus hide range along
> with shadow register values set by BIOS and perform a bus scan.
> 
> The commit also involves small refactoring of vmd_enable_domain function.
> ...

> +static int vmd_prepare_offsets_and_bus(struct vmd_dev *vmd,
> +					unsigned long features,
> +					resource_size_t *membar2_offset,
> +					resource_size_t *offset1,
> +					resource_size_t *offset2)
> +{
> +	int ret;
> +
> +	/*
> +	 * Shadow registers may exist in certain VMD device ids which allow
> +	 * guests to correctly assign host physical addresses to the root ports
> +	 * and child devices. These registers will either return the host value
> +	 * or 0, depending on an enable bit in the VMD device.
> +	 *
> +	 * For certain VMD devices (i.e. 0x28C1), BIOS places device info
> +	 * in BAR4 shadow registers to determine the base bus number and memory
> +	 * offsets.
> +	 */
> +	if (features & VMD_FEAT_USE_BIOS_INFO) {
> +		*membar2_offset = MEMBAR2_OFFSET_28C1;
> +		ret = vmd_get_bus_info_from_bar4(vmd, offset1, offset2);

Is it guaranteed that for device 0x28C1, BIOS will *always* do the
enumeration?  Sounds like a rev-lock between this device and the BIOS,
i.e., this device can't be used at all with an old BIOS that doesn't
know how to do that enumeration?

> @@ -1114,6 +1228,10 @@ static const struct pci_device_id vmd_ids[] = {
>  		.driver_data = VMD_FEAT_HAS_MEMBAR_SHADOW |
>  				VMD_FEAT_HAS_BUS_RESTRICTIONS |
>  				VMD_FEAT_CAN_BYPASS_MSI_REMAP,},
> +	{PCI_VDEVICE(INTEL, PCI_DEVICE_ID_INTEL_VMD_28C1),
> +		.driver_data = VMD_FEAT_HAS_MEMBAR_SHADOW |
> +				VMD_FEAT_CAN_BYPASS_MSI_REMAP |
> +				VMD_FEAT_USE_BIOS_INFO,},
>  	{PCI_VDEVICE(INTEL, 0x467f),
>  		.driver_data = VMD_FEATS_CLIENT,},
>  	{PCI_VDEVICE(INTEL, 0x4c3d),