linux-kernel - Re: [PATCH v0 10/15] PCI: hv: Build device id for a VMBus device

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Message-ID: <aXAAH4G9ztAGDWuy@skinsburskii.localdomain>
Date: Tue, 20 Jan 2026 14:22:23 -0800
From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
To: Mukesh R <mrathor@...ux.microsoft.com>
Cc: linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-pci@...r.kernel.org, linux-arch@...r.kernel.org,
	kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
	decui@...rosoft.com, longli@...rosoft.com, catalin.marinas@....com,
	will@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, hpa@...or.com, joro@...tes.org,
	lpieralisi@...nel.org, kwilczynski@...nel.org, mani@...nel.org,
	robh@...nel.org, bhelgaas@...gle.com, arnd@...db.de,
	nunodasneves@...ux.microsoft.com, mhklinux@...look.com,
	romank@...ux.microsoft.com
Subject: Re: [PATCH v0 10/15] PCI: hv: Build device id for a VMBus device

On Mon, Jan 19, 2026 at 10:42:25PM -0800, Mukesh R wrote:
> From: Mukesh Rathor <mrathor@...ux.microsoft.com>
> 
> On Hyper-V, most hypercalls related to PCI passthru to map/unmap regions,
> interrupts, etc need a device id as a parameter. This device id refers
> to that specific device during the lifetime of passthru.
> 
> An L1VH VM only contains VMBus based devices. A device id for a VMBus
> device is slightly different in that it uses the hv_pcibus_device info
> for building it to make sure it matches exactly what the hypervisor
> expects. This VMBus based device id is needed when attaching devices in
> an L1VH based guest VM. Before building it, a check is done to make sure
> the device is a valid VMBus device.
> 
> Signed-off-by: Mukesh Rathor <mrathor@...ux.microsoft.com>
> ---
>  arch/x86/include/asm/mshyperv.h     |  2 ++
>  drivers/pci/controller/pci-hyperv.c | 29 +++++++++++++++++++++++++++++
>  2 files changed, 31 insertions(+)
> 
> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
> index eef4c3a5ba28..0d7fdfb25e76 100644
> --- a/arch/x86/include/asm/mshyperv.h
> +++ b/arch/x86/include/asm/mshyperv.h
> @@ -188,6 +188,8 @@ bool hv_vcpu_is_preempted(int vcpu);
>  static inline void hv_apic_init(void) {}
>  #endif
>  
> +u64 hv_pci_vmbus_device_id(struct pci_dev *pdev);
> +
>  struct irq_domain *hv_create_pci_msi_domain(void);
>  
>  int hv_map_msi_interrupt(struct irq_data *data,
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index 8bc6a38c9b5a..40f0b06bb966 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -579,6 +579,8 @@ static void hv_pci_onchannelcallback(void *context);
>  #define DELIVERY_MODE		APIC_DELIVERY_MODE_FIXED
>  #define HV_MSI_CHIP_FLAGS	MSI_CHIP_FLAG_SET_ACK
>  
> +static bool hv_vmbus_pci_device(struct pci_bus *pbus);
> +

Why not moving this static function definition above the called instead of
defining the prototype?

>  static int hv_pci_irqchip_init(void)
>  {
>  	return 0;
> @@ -598,6 +600,26 @@ static unsigned int hv_msi_get_int_vector(struct irq_data *data)
>  
>  #define hv_msi_prepare		pci_msi_prepare
>  
> +u64 hv_pci_vmbus_device_id(struct pci_dev *pdev)
> +{
> +	u64 u64val;

This variable is redundant.

> +	struct hv_pcibus_device *hbus;
> +	struct pci_bus *pbus = pdev->bus;
> +
> +	if (!hv_vmbus_pci_device(pbus))
> +		return 0;
> +
> +	hbus = container_of(pbus->sysdata, struct hv_pcibus_device, sysdata);
> +	u64val = (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);
> +

It looks like this value always fits into 32 bit, so what is the value
in returning 64 bit?

Thanks,
Stanislav

> +	return u64val;
> +}
> +EXPORT_SYMBOL_GPL(hv_pci_vmbus_device_id);
> +
>  /**
>   * hv_irq_retarget_interrupt() - "Unmask" the IRQ by setting its current
>   * affinity.
> @@ -1404,6 +1426,13 @@ static struct pci_ops hv_pcifront_ops = {
>  	.write = hv_pcifront_write_config,
>  };
>  
> +#ifdef CONFIG_X86
> +static bool hv_vmbus_pci_device(struct pci_bus *pbus)
> +{
> +	return pbus->ops == &hv_pcifront_ops;
> +}
> +#endif /* CONFIG_X86 */
> +
>  /*
>   * Paravirtual backchannel
>   *
> -- 
> 2.51.2.vfs.0.1
>