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

[Wei Liu <wei.liu@kernel.org>]

On Thu, Sep 17, 2026 at 11:01:03AM +0530, Sahil Chandna wrote:
> On 07-09-2026 11:17, Naman Jain wrote:
> > 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.
> > 
> > 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)
> LGTM.
> Reviewed-by: Sahil Chandna <sahilchandna@linux.microsoft.com>

Applied. Thanks.