RE: [PATCH] PCI: hv: Probe vPCI buses asynchronously

[Michael Kelley <mhklinux@outlook.com>]

From: Naman Jain <namjain@linux.microsoft.com> Sent: Sunday, September 6, 2026 10:48 PM
> 
> On Hyper-V guests each virtual PCI bus is enumerated by its own
> hv_pci_probe() call. The probe performs several synchronous host
> request/response exchanges while negotiating the protocol, querying bus
> relations, entering D0, and reporting allocated resources. These waits
> are latency-bound rather than CPU-bound.
> 
> hv_pci registers as an ordinary VMBus driver, so driver_register() walks
> matching vPCI buses and probes them sequentially while the driver's
> initcall runs. On guests that expose several devices, each through its
> own vPCI bus, this serialization adds the host round-trip latencies to
> device initialization.
> 
> Each bus is described by its own struct hv_pcibus_device, so
> independent buses can be probed concurrently. Request asynchronous
> probing via PROBE_PREFER_ASYNCHRONOUS, causing the driver core to
> schedule matching buses for asynchronous probe work.
> 
> On an Azure Standard_L32s_v3 guest with five vPCI targets (four NVMe
> controllers and one Mellanox VF), Linux 7.2.3 was tested with one warm-up
> and three measured boots per variant. The median interval from the first
> hv_pci_probe() entry to the last return decreased from 2847.968 ms to
> 2786.709 ms, a 61.259 ms (2.15%) improvement.

The elapsed time improvement is rather disappointing given the
complexity of the probing sequence and the number of interactions
with the Hyper-V host. Do you have any insight into why there isn't a
larger reduction? Is something mostly serializing the work even though
PROBE_PREFER_ASYNCHRONOUS is specified?

Michael

> 
> Co-developed-by: Dexuan Cui <decui@microsoft.com>
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
> ---
> 
> Previous discussion around this change:
> https://lore.kernel.org/all/20230420024037.5921-7-decui@microsoft.com/
> 
> Skipping removal of pci_rescan_remove_lock, due to possible
> synchronization problems associated with this lock removal. Also, with
> my current setup, I was not able to see much improvements with this
> change, so keeping it for later. Asynchronous probing change is not
> dependent on this.
> 
> ---
>  drivers/pci/controller/pci-hyperv.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index 89816a2bd7cd3..056d379b3cee4 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -4155,6 +4155,9 @@ static struct hv_driver hv_pci_drv = {
>  	.remove		= hv_pci_remove,
>  	.suspend	= hv_pci_suspend,
>  	.resume		= hv_pci_resume,
> +	.driver = {
> +		.probe_type = PROBE_PREFER_ASYNCHRONOUS,
> +	},
>  };
> 
>  static void __exit exit_hv_pci_drv(void)
> --
> 2.43.0
>