[PATCH] PCI: hv: Allocate MMIO from above 4GB for the config window

[Dexuan Cui <decui@microsoft.com>]

There has been a longstanding MMIO conflict between the pci_hyperv
driver's config_window (see hv_allocate_config_window()) and the
hyperv_drm (or hyperv_fb) driver (see hyperv_setup_vram()): typically
both get MMIO from the low MMIO range below 4GB; this is not an issue
in the normal kernel since the VMBus driver reserves the framebuffer
MMIO in vmbus_reserve_fb(), so the drm driver's hyperv_setup_vram() can
always get the reserved framebuffer MMIO; however, a Gen2 VM's kdump
kernel fails to reserve the framebuffer MMIO in vmbus_reserve_fb() because
the screen_info.lfb_base is zero in the kdump kernel: the screen_info
is not initialized at all in the kdump kernel, because the EFI stub
code, which initializes screen_info, doesn't run in the case of kdump.

When vmbus_reserve_fb() fails to reserve the framebuffer MMIO in the
kdump kernel, if pci_hyperv in the kdump kernel loads before hyperv_drm
loads, pci_hyperv's vmbus_allocate_mmio() gets the framebuffer MMIO
and tries to use it, but since the host thinks that the MMIO range is
still in use by hyperv_drm, the host refuses to accept the MMIO range
as the config window, and pci_hyperv's hv_pci_enter_d0() errors out:
"PCI Pass-through VSP failed D0 Entry with status c0370048".

This PCI error in the kdump kernel was not fatal in the past because
the kdump kernel normally doesn't reply on pci_hyperv, and the root
file system is on a VMBus SCSI device.

Now, a VM on Azure can boot from NVMe, i.e. the root FS can be on a
NVMe device, which depends on pci_hyperv. When the PCI error occurs,
the kdump kernel fails to boot up since no root FS is detected.

Fix the MMIO conflict by allocating MMIO above 4GB for the
config_window.

Note: we still need to figure out how to address the possible MMIO
conflict between hyperv_drm and pci_hyperv in the case of 32-bit PCI
MMIO BARs, but that's of low priority because all PCI devices available
to a Linux VM on Azure should use 64-bit BARs and should not use 32-bit
BARs -- I checked Mellanox VFs, MANA VFs, NVMe devices, and GPUs in
Linux VMs on Azure, and found no 32-bit BARs.

Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: stable@vger.kernel.org
---
 drivers/pci/controller/pci-hyperv.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index 1e237d3538f9..a6aecb1b5cab 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -3406,9 +3406,13 @@ static int hv_allocate_config_window(struct hv_pcibus_device *hbus)
 
 	/*
 	 * Set up a region of MMIO space to use for accessing configuration
-	 * space.
+	 * space. Use the high MMIO range to not conflict with the hyperv_drm
+	 * driver (which normally gets MMIO from the low MMIO range) in the
+	 * kdump kernel of a Gen2 VM, which fails to reserve the framebuffer
+	 * MMIO range in vmbus_reserve_fb() due to screen_info.lfb_base being
+	 * zero in the kdump kernel.
 	 */
-	ret = vmbus_allocate_mmio(&hbus->mem_config, hbus->hdev, 0, -1,
+	ret = vmbus_allocate_mmio(&hbus->mem_config, hbus->hdev, SZ_4G, -1,
 				  PCI_CONFIG_MMIO_LENGTH, 0x1000, false);
 	if (ret)
 		return ret;
-- 
2.43.0