linux-kernel - Re: [RFC v1 1/5] PCI: hv: Create and export hv_build_logical_dev

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Message-ID: <20251210213945.GA3541010@bhelgaas>
Date: Wed, 10 Dec 2025 15:39:45 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Yu Zhang <zhangyu1@...ux.microsoft.com>
Cc: linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
	iommu@...ts.linux.dev, linux-pci@...r.kernel.org, kys@...rosoft.com,
	haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com,
	lpieralisi@...nel.org, kwilczynski@...nel.org, mani@...nel.org,
	robh@...nel.org, bhelgaas@...gle.com, arnd@...db.de,
	joro@...tes.org, will@...nel.org, robin.murphy@....com,
	easwar.hariharan@...ux.microsoft.com, jacob.pan@...ux.microsoft.com,
	nunodasneves@...ux.microsoft.com, mrathor@...ux.microsoft.com,
	mhklinux@...look.com, peterz@...radead.org,
	linux-arch@...r.kernel.org
Subject: Re: [RFC v1 1/5] PCI: hv: Create and export hv_build_logical_dev_id()

On Tue, Dec 09, 2025 at 01:11:24PM +0800, Yu Zhang wrote:
> From: Easwar Hariharan <easwar.hariharan@...ux.microsoft.com>
> 
> Hyper-V uses a logical device ID to identify a PCI endpoint device for
> child partitions. This ID will also be required for future hypercalls
> used by the Hyper-V IOMMU driver.
> 
> Refactor the logic for building this logical device ID into a standalone
> helper function and export the interface for wider use.
> 
> Signed-off-by: Easwar Hariharan <easwar.hariharan@...ux.microsoft.com>
> Signed-off-by: Yu Zhang <zhangyu1@...ux.microsoft.com>

Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>

> ---
>  drivers/pci/controller/pci-hyperv.c | 28 ++++++++++++++++++++--------
>  include/asm-generic/mshyperv.h      |  2 ++
>  2 files changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index 146b43981b27..4b82e06b5d93 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -598,15 +598,31 @@ static unsigned int hv_msi_get_int_vector(struct irq_data *data)
>  
>  #define hv_msi_prepare		pci_msi_prepare
>  
> +/**
> + * Build a "Device Logical ID" out of this PCI bus's instance GUID and the
> + * function number of the device.
> + */
> +u64 hv_build_logical_dev_id(struct pci_dev *pdev)
> +{
> +	struct pci_bus *pbus = pdev->bus;
> +	struct hv_pcibus_device *hbus = container_of(pbus->sysdata,
> +						struct hv_pcibus_device, sysdata);
> +
> +	return (u64)((hbus->hdev->dev_instance.b[5] << 24) |
> +		     (hbus->hdev->dev_instance.b[4] << 16) |
> +		     (hbus->hdev->dev_instance.b[7] << 8)  |
> +		     (hbus->hdev->dev_instance.b[6] & 0xf8) |
> +		     PCI_FUNC(pdev->devfn));
> +}
> +EXPORT_SYMBOL_GPL(hv_build_logical_dev_id);
> +
>  /**
>   * hv_irq_retarget_interrupt() - "Unmask" the IRQ by setting its current
>   * affinity.
>   * @data:	Describes the IRQ
>   *
>   * Build new a destination for the MSI and make a hypercall to
> - * update the Interrupt Redirection Table. "Device Logical ID"
> - * is built out of this PCI bus's instance GUID and the function
> - * number of the device.
> + * update the Interrupt Redirection Table.
>   */
>  static void hv_irq_retarget_interrupt(struct irq_data *data)
>  {
> @@ -642,11 +658,7 @@ static void hv_irq_retarget_interrupt(struct irq_data *data)
>  	params->int_entry.source = HV_INTERRUPT_SOURCE_MSI;
>  	params->int_entry.msi_entry.address.as_uint32 = int_desc->address & 0xffffffff;
>  	params->int_entry.msi_entry.data.as_uint32 = int_desc->data;
> -	params->device_id = (hbus->hdev->dev_instance.b[5] << 24) |
> -			   (hbus->hdev->dev_instance.b[4] << 16) |
> -			   (hbus->hdev->dev_instance.b[7] << 8) |
> -			   (hbus->hdev->dev_instance.b[6] & 0xf8) |
> -			   PCI_FUNC(pdev->devfn);
> +	params->device_id = hv_build_logical_dev_id(pdev);
>  	params->int_target.vector = hv_msi_get_int_vector(data);
>  
>  	if (hbus->protocol_version >= PCI_PROTOCOL_VERSION_1_2) {
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index 64ba6bc807d9..1a205ed69435 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -71,6 +71,8 @@ extern enum hv_partition_type hv_curr_partition_type;
>  extern void * __percpu *hyperv_pcpu_input_arg;
>  extern void * __percpu *hyperv_pcpu_output_arg;
>  
> +extern u64 hv_build_logical_dev_id(struct pci_dev *pdev);

Curious why you would include the "extern" in this declaration?  It's
not *wrong*, but it's not necessary, and other declarations in this
file omit it, e.g., the ones below:

>  u64 hv_do_hypercall(u64 control, void *inputaddr, void *outputaddr);
>  u64 hv_do_fast_hypercall8(u16 control, u64 input8);
>  u64 hv_do_fast_hypercall16(u16 control, u64 input1, u64 input2);
> -- 
> 2.49.0
>