5.15-stable review patch. If anyone has any objections, please let me know.
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From: Dexuan Cui decui@microsoft.com
commit 23e118a48acf7be223e57d98e98da8ac5a4071ac upstream.
Currently when the pci-hyperv driver finishes probing and initializing the PCI device, it sets the PCI_COMMAND_MEMORY bit; later when the PCI device is registered to the core PCI subsystem, the core PCI driver's BAR detection and initialization code toggles the bit multiple times, and each toggling of the bit causes the hypervisor to unmap/map the virtual BARs from/to the physical BARs, which can be slow if the BAR sizes are huge, e.g., a Linux VM with 14 GPU devices has to spend more than 3 minutes on BAR detection and initialization, causing a long boot time.
Reduce the boot time by not setting the PCI_COMMAND_MEMORY bit when we register the PCI device (there is no need to have it set in the first place). The bit stays off till the PCI device driver calls pci_enable_device(). With this change, the boot time of such a 14-GPU VM is reduced by almost 3 minutes.
Link: https://lore.kernel.org/lkml/20220419220007.26550-1-decui@microsoft.com/ Tested-by: Boqun Feng (Microsoft) boqun.feng@gmail.com Signed-off-by: Dexuan Cui decui@microsoft.com Reviewed-by: Michael Kelley mikelley@microsoft.com Acked-by: Lorenzo Pieralisi lorenzo.pieralisi@arm.com Cc: Jake Oshins jakeo@microsoft.com Link: https://lore.kernel.org/r/20220502074255.16901-1-decui@microsoft.com Signed-off-by: Wei Liu wei.liu@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/pci/controller/pci-hyperv.c | 17 +++++++++++------ 1 file changed, 11 insertions(+), 6 deletions(-)
--- a/drivers/pci/controller/pci-hyperv.c +++ b/drivers/pci/controller/pci-hyperv.c @@ -1882,12 +1882,17 @@ static void prepopulate_bars(struct hv_p } } if (high_size <= 1 && low_size <= 1) { - /* Set the memory enable bit. */ - _hv_pcifront_read_config(hpdev, PCI_COMMAND, 2, - &command); - command |= PCI_COMMAND_MEMORY; - _hv_pcifront_write_config(hpdev, PCI_COMMAND, 2, - command); + /* + * No need to set the PCI_COMMAND_MEMORY bit as + * the core PCI driver doesn't require the bit + * to be pre-set. Actually here we intentionally + * keep the bit off so that the PCI BAR probing + * in the core PCI driver doesn't cause Hyper-V + * to unnecessarily unmap/map the virtual BARs + * from/to the physical BARs multiple times. + * This reduces the VM boot time significantly + * if the BAR sizes are huge. + */ break; } }