Re: [PATCH v4 2/5] Drivers: hv: Add logical device ID registry for vPCI devices

> Hyper-V identifies each PCI pass-thru device by a logical device ID in
> its hypercall interface. This ID consists of a per-bus prefix, derived
> from the VMBus device instance GUID, combined with the PCI function
> number of the endpoint device.
> 
> Add a registry in hv_common.c that maps a PCI domain number to its
> logical device ID prefix. The vPCI bus driver (pci-hyperv) registers the
> prefix when a bus is probed and unregisters it when the bus is removed.
> Consumers such as the para-virtualized IOMMU driver look up the prefix
> by PCI domain number and combine it with the function number to form the
> complete logical device ID for hypercalls.
> 
> Use rhashtable for the sparse exact-match mapping. Lookups copy the
> prefix while holding the RCU read lock, and removal defers freeing the
> entry until existing readers have completed.
> 
> The prefix construction is shared via hv_build_logical_dev_id_prefix() so
> that pci-hyperv's interrupt retargeting path and the registry use exactly
> the same byte layout. It is derived on demand from the constant hv_device
> instance GUID rather than cached in struct hv_pcibus_device, which is
> private to the pci-hyperv module; this keeps the interface narrow and
> avoids depending on pci-hyperv internals.
> 
> Co-developed-by: Easwar Hariharan <[email protected]>
> Signed-off-by: Easwar Hariharan <[email protected]>
> Signed-off-by: Yu Zhang <[email protected]>

Sashiko has reviewed this patch and found no issues. It looks great!

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