linux-kernel - RE: [PATCH] PCI: hv: Do not set PCI_COMMAND

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Message-ID: <BY3PR21MB3033800D4F4A1D0C94B81290D7F59@BY3PR21MB3033.namprd21.prod.outlook.com>
Date:   Wed, 20 Apr 2022 14:47:43 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Dexuan Cui <decui@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh@...nel.org" <robh@...nel.org>, "kw@...ux.com" <kw@...ux.com>
CC:     Jake Oshins <jakeo@...rosoft.com>
Subject: RE: [PATCH] PCI: hv: Do not set PCI_COMMAND_MEMORY to reduce VM boot
 time

From: Dexuan Cui <decui@...rosoft.com> Sent: Tuesday, April 19, 2022 3:00 PM
> 
> A VM on Azure can have 14 GPUs, and each GPU may have a huge MMIO BAR,
> e.g. 128 GB. Currently the boot time of such a VM can be 4+ minutes, and
> most of the time is used by the host to unmap/map the vBAR from/to pBAR
> when the VM clears and sets the PCI_COMMAND_MEMORY bit: each unmap/map
> operation for a 128GB BAR needs about 1.8 seconds, and the pci-hyperv
> driver and the Linux PCI subsystem flip the PCI_COMMAND_MEMORY bit
> eight times (see pci_setup_device() -> pci_read_bases() and
> pci_std_update_resource()), increasing the boot time by 1.8 * 8 = 14.4
> seconds per GPU, i.e. 14.4 * 14 = 201.6 seconds in total.
> 
> Fix the slowness by not turning on the PCI_COMMAND_MEMORY in pci-hyperv.c,
> so the bit stays in the off state before the PCI device driver calls
> pci_enable_device(): when the bit is off, pci_read_bases() and
> pci_std_update_resource() don't cause Hyper-V to unmap/map the vBARs.
> With this change, the boot time of such a VM is reduced by
> 1.8 * (8-1) * 14 = 176.4 seconds.
> 
> Tested-by: Boqun Feng (Microsoft) <boqun.feng@...il.com>
> Signed-off-by: Dexuan Cui <decui@...rosoft.com>
> Cc: Jake Oshins <jakeo@...rosoft.com>
> ---
>  drivers/pci/controller/pci-hyperv.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index d270a204324e..f9fbbd8d94db 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -2082,12 +2082,17 @@ static void prepopulate_bars(struct hv_pcibus_device
> *hbus)
>  				}
>  			}
>  			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;
>  			}
>  		}
> --
> 2.17.1

Reviewed-by: Michael Kelley <mikelley@...rosoft.com>