Re: [PATCH v4] PCI: vmd: Add feature to scan BIOS enumerated devices.

[Nirmal Patel <nirmal.patel@linux.intel.com>]

On Fri, 22 May 2026 15:08:29 +0000
Nirmal Patel <nirmal.patel@linux.intel.com> 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 patch also involves small refactoring of vmd_enable_domain
> function.
> 
> Signed-off-by: Nirmal Patel <nirmal.patel@linux.intel.com>
> ---
> v4 : Updating vmd_set_msi_remapping for supported devices only.
> v3 : Hard code configbar .end to 0xff same as probe.c; Adjust membar2
>      offset to accommodate more registers in 28C1. Remove redundant
>      IORESOURCE_MEM check in vmd_prepare_offsets_and_bus.
> v2 : Using PCI features flag instead of devic ID and fixing corner
> cases for vmd_remove_irq.
> ---
>  drivers/pci/controller/vmd.c | 174
> +++++++++++++++++++++++++++-------- include/linux/pci_ids.h      |
> 1 + 2 files changed, 136 insertions(+), 39 deletions(-)
> 
> diff --git a/drivers/pci/controller/vmd.c
> b/drivers/pci/controller/vmd.c index d4ae250d4bc6..7c90243b5ff0 100644
> --- a/drivers/pci/controller/vmd.c
> +++ b/drivers/pci/controller/vmd.c
> @@ -37,6 +37,12 @@
>  #define MB2_SHADOW_OFFSET	0x2000
>  #define MB2_SHADOW_SIZE		16
>  
> +/* DMR BAR4 register offsets */
> +#define SHADOW_MEMBAR1_28C1		0x2818 /* MEMBAR1
> physical address */ +#define SHADOW_MEMBAR2_28C1
> 0x2820 /* MEMBAR2 physical address */ +#define
> BASE_ID_REG_28C1		0x2840 +#define
> MEMBAR2_OFFSET_28C1		0x30d0 +
>  enum vmd_features {
>  	/*
>  	 * Device may contain registers which hint the physical
> location of the @@ -77,6 +83,15 @@ enum vmd_features {
>  	 * proper power management of the SoC.
>  	 */
>  	VMD_FEAT_BIOS_PM_QUIRK		= (1 << 5),
> +
> +	/*
> +	 * Newer VMD  with device ID 0x28c1 has unique settings
> compared to its
> +	 * predecessor where BIOS enumerates the entire VMD device
> tree and
> +	 * stores respective configurations including bus start
> range and
> +	 * shadow registers in VMD MMIO space in VMD BAR4/BAR5,
> otherwise refers
> +	 * to as MEMBAR2 or MSI-X bar.
> +	 */
> +	VMD_FEAT_USE_BIOS_INFO		= (1 << 6),
>  };
>  
>  #define VMD_BIOS_PM_QUIRK_LTR	0x1003	/* 3145728 ns */
> @@ -142,6 +157,7 @@ struct vmd_dev {
>  	u8			first_vec;
>  	char			*name;
>  	int			instance;
> +	unsigned long		features;
>  };
>  
>  static inline struct vmd_dev *vmd_from_bus(struct pci_bus *bus)
> @@ -365,6 +381,8 @@ static int vmd_create_irq_domain(struct vmd_dev
> *vmd) static void vmd_set_msi_remapping(struct vmd_dev *vmd, bool
> enable) {
>  	u16 reg;
> +	if (!!(vmd->features & VMD_FEAT_USE_BIOS_INFO))
> +		return;
>  
>  	pci_read_config_word(vmd->dev, PCI_REG_VMCONFIG, &reg);
>  	reg = enable ? (reg & ~VMCONFIG_MSI_REMAP) :
> @@ -374,6 +392,8 @@ static void vmd_set_msi_remapping(struct vmd_dev
> *vmd, bool enable) 
>  static void vmd_remove_irq_domain(struct vmd_dev *vmd)
>  {
> +	if (!!(vmd->features & VMD_FEAT_USE_BIOS_INFO))
> +		return;
>  	/*
>  	 * Some production BIOS won't enable remapping between soft
> reboots.
>  	 * Ensure remapping is restored before unloading the driver.
> @@ -393,7 +413,12 @@ static void __iomem *vmd_cfg_addr(struct vmd_dev
> *vmd, struct pci_bus *bus, unsigned int devfn, int reg, int len)
>  {
>  	unsigned int busnr_ecam = bus->number - vmd->busn_start;
> -	u32 offset = PCIE_ECAM_OFFSET(busnr_ecam, devfn, reg);
> +	u32 offset;
> +
> +	if (!!(vmd->features & VMD_FEAT_USE_BIOS_INFO))
> +		busnr_ecam = bus->number;
> +
> +	offset = PCIE_ECAM_OFFSET(busnr_ecam, devfn, reg);
>  
>  	if (offset + len >=
> resource_size(&vmd->dev->resource[VMD_CFGBAR])) return NULL;
> @@ -661,6 +686,46 @@ static int vmd_get_bus_number_start(struct
> vmd_dev *vmd) return 0;
>  }
>  
> +static int vmd_get_bus_info_from_bar4(struct vmd_dev *vmd,
> +				       resource_size_t *offset1,
> +				       resource_size_t *offset2)
> +{
> +	u64 phys1, phys2, bar4_2840;
> +	void __iomem *bar4;
> +	u32 base_id;
> +	u8 base_bus;
> +
> +
> +	bar4 = pci_ioremap_bar(vmd->dev, 4);
> +	if (!bar4)
> +		return -ENOMEM;
> +
> +	/* Read shadow registers for MEMBAR1 and MEMBAR2 physical
> addresses. */
> +	phys1 = readq(bar4 + SHADOW_MEMBAR1_28C1);
> +	phys2 = readq(bar4 + SHADOW_MEMBAR2_28C1);
> +
> +	/*
> +	 * Read and set bus start number from Base ID register.
> +	 * 24-bit Base ID register is part of 64-bit shadowed reqid
> hide
> +	 * range register and holds segment, bus, device and
> function.
> +	 */
> +	bar4_2840 = readq(bar4 + BASE_ID_REG_28C1);
> +	base_id = bar4_2840 & 0xFFFFFF;
> +	base_bus = base_id >> 8;
> +	vmd->busn_start = base_bus;
> +
> +	/* Calculate offsets like vmd_get_phys_offsets() does. */
> +	if (phys1)
> +		*offset1 = vmd->dev->resource[VMD_MEMBAR1].start -
> +			(phys1 & PCI_BASE_ADDRESS_MEM_MASK);
> +	if (phys2)
> +		*offset2 = vmd->dev->resource[VMD_MEMBAR2].start -
> +			(phys2 & PCI_BASE_ADDRESS_MEM_MASK);
> +
> +	pci_iounmap(vmd->dev, bar4);
> +	return 0;
> +}
> +
>  static irqreturn_t vmd_irq(int irq, void *data)
>  {
>  	struct vmd_irq_list *irqs = data;
> @@ -711,6 +776,53 @@ static int vmd_alloc_irqs(struct vmd_dev *vmd)
>  	return 0;
>  }
>  
> +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);
> +		if (ret)
> +			return ret;
> +	} else if (features & VMD_FEAT_HAS_MEMBAR_SHADOW) {
> +		*membar2_offset = MB2_SHADOW_OFFSET +
> MB2_SHADOW_SIZE;
> +		ret = vmd_get_phys_offsets(vmd, true, offset1,
> offset2);
> +		if (ret)
> +			return ret;
> +	} else if (features & VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP) {
> +		ret = vmd_get_phys_offsets(vmd, false, offset1,
> offset2);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	/*
> +	 * Certain VMD devices may have a root port configuration
> option which
> +	 * limits the bus range to between 0-127, 128-255, or
> 224-255.
> +	 */
> +	if (features & VMD_FEAT_HAS_BUS_RESTRICTIONS) {
> +		ret = vmd_get_bus_number_start(vmd);
> +		if (ret)
> +			return ret;
> +	}
> +	return 0;
> +}
> +
>  /*
>   * Since VMD is an aperture to regular PCIe root ports, only allow
> it to
>   * control features that the OS is allowed to control on the
> physical PCI bus. @@ -784,38 +896,16 @@ static int
> vmd_enable_domain(struct vmd_dev *vmd, unsigned long features) struct
> pci_dev *dev; 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.
> -	 */
> -	if (features & VMD_FEAT_HAS_MEMBAR_SHADOW) {
> -		membar2_offset = MB2_SHADOW_OFFSET + MB2_SHADOW_SIZE;
> -		ret = vmd_get_phys_offsets(vmd, true, &offset[0],
> &offset[1]);
> -		if (ret)
> -			return ret;
> -	} else if (features & VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP) {
> -		ret = vmd_get_phys_offsets(vmd, false, &offset[0],
> &offset[1]);
> -		if (ret)
> -			return ret;
> -	}
> -
> -	/*
> -	 * Certain VMD devices may have a root port configuration
> option which
> -	 * limits the bus range to between 0-127, 128-255, or 224-255
> -	 */
> -	if (features & VMD_FEAT_HAS_BUS_RESTRICTIONS) {
> -		ret = vmd_get_bus_number_start(vmd);
> -		if (ret)
> -			return ret;
> -	}
> +	ret = vmd_prepare_offsets_and_bus(vmd, features,
> &membar2_offset,
> +					  &offset[0], &offset[1]);
> +	if (ret)
> +		return ret;
>  
>  	res = &vmd->dev->resource[VMD_CFGBAR];
>  	vmd->resources[0] = (struct resource) {
>  		.name  = "VMD CFGBAR",
>  		.start = vmd->busn_start,
> -		.end   = vmd->busn_start + (resource_size(res) >>
> 20) - 1,
> +		.end   = 0xff,
>  		.flags = IORESOURCE_BUS | IORESOURCE_PCI_FIXED,
>  	};
>  
> @@ -868,19 +958,21 @@ static int vmd_enable_domain(struct vmd_dev
> *vmd, unsigned long features)
>  	 * acceptable because the guest is usually CPU-limited and
> MSI
>  	 * remapping doesn't become a performance bottleneck.
>  	 */
> -	if (!(features & VMD_FEAT_CAN_BYPASS_MSI_REMAP) ||
> -	    offset[0] || offset[1]) {
> -		ret = vmd_alloc_irqs(vmd);
> -		if (ret)
> -			return ret;
> +	if (!(features & VMD_FEAT_USE_BIOS_INFO)) {
> +		if (!(features & VMD_FEAT_CAN_BYPASS_MSI_REMAP) ||
> +		    offset[0] || offset[1]) {
> +			ret = vmd_alloc_irqs(vmd);
> +			if (ret)
> +				return ret;
>  
> -		vmd_set_msi_remapping(vmd, true);
> +			vmd_set_msi_remapping(vmd, true);
>  
> -		ret = vmd_create_irq_domain(vmd);
> -		if (ret)
> -			return ret;
> -	} else {
> -		vmd_set_msi_remapping(vmd, false);
> +			ret = vmd_create_irq_domain(vmd);
> +			if (ret)
> +				return ret;
> +		} else {
> +			vmd_set_msi_remapping(vmd, false);
> +		}
>  	}
>  
>  	pci_add_resource(&resources, &vmd->resources[0]);
> @@ -998,6 +1090,7 @@ static int vmd_probe(struct pci_dev *dev, const
> struct pci_device_id *id) 
>  	vmd->dev = dev;
>  	vmd->sysdata.domain = PCI_DOMAIN_NR_NOT_SET;
> +	vmd->features = features;
>  	vmd->instance = ida_alloc(&vmd_instance_ida, GFP_KERNEL);
>  	if (vmd->instance < 0)
>  		return vmd->instance;
> @@ -1114,6 +1207,9 @@ 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_USE_BIOS_INFO,},
>  	{PCI_VDEVICE(INTEL, 0x467f),
>  		.driver_data = VMD_FEATS_CLIENT,},
>  	{PCI_VDEVICE(INTEL, 0x4c3d),
> diff --git a/include/linux/pci_ids.h b/include/linux/pci_ids.h
> index 24cb42f66e4b..2a8ebe7df92e 100644
> --- a/include/linux/pci_ids.h
> +++ b/include/linux/pci_ids.h
> @@ -2889,6 +2889,7 @@
>  #define PCI_DEVICE_ID_INTEL_HDA_ICH8	0x284b
>  #define PCI_DEVICE_ID_INTEL_ICH8_6	0x2850
>  #define PCI_DEVICE_ID_INTEL_VMD_28C0	0x28c0
> +#define PCI_DEVICE_ID_INTEL_VMD_28C1    0x28c1
>  #define PCI_DEVICE_ID_INTEL_ICH9_0	0x2910
>  #define PCI_DEVICE_ID_INTEL_ICH9_2	0x2912
>  #define PCI_DEVICE_ID_INTEL_ICH9_3	0x2913

Gentle reminder. FYI, I also addressed comments from AI review.

Thanks.