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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 19 Apr 2022 15:00:07 -0700
From:   Dexuan Cui <decui@...rosoft.com>
To:     wei.liu@...nel.org, kys@...rosoft.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, lorenzo.pieralisi@....com,
        bhelgaas@...gle.com, linux-hyperv@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        mikelley@...rosoft.com, robh@...nel.org, kw@...ux.com
Cc:     jakeo@...rosoft.com, Dexuan Cui <decui@...rosoft.com>
Subject: [PATCH] PCI: hv: Do not set PCI_COMMAND_MEMORY to reduce VM boot time

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