> From: Michael Kelley <[email protected]> > Sent: Wednesday, May 6, 2026 8:14 AM > > ... > > + /* > > + * If the kdump kernel's lfb_base is 0, > > Nit: The case of lfb_base is 0 applies to kexec and kdump kernels, and also > to > CVMs.
Thanks for catching this! I'm going to post this v3 later today. --- v2-0001-Drivers-hv-vmbus-Improve-the-logic-of-reserving-fb_m.patch 2026-05-04 17:48:23.486911073 -0700 +++ v3-0001-Drivers-hv-vmbus-Improve-the-logic-of-reserving-fb_m.patch 2026-05-06 18:03:42.922469286 -0700 @@ -1,15 +1,15 @@ From 5d817788d65febdc0451e8a88277778794fe87b2 Mon Sep 17 00:00:00 2001 From: Dexuan Cui <[email protected]> Date: Thu, 16 Apr 2026 04:30:21 +0000 -Subject: [PATCH v2] Drivers: hv: vmbus: Improve the logic of reserving fb_mmio on +Subject: [PATCH v3] Drivers: hv: vmbus: Improve the logic of reserving fb_mmio on Gen2 VMs If vmbus_reserve_fb() in the kdump/kexec kernel fails to properly reserve the framebuffer MMIO range (which is below 4GB) due to a Gen2 VM's screen.lfb_base being zero [1], there is an MMIO conflict between the drivers hyperv-drm and pci-hyperv: when the driver pci-hyperv's -hv_pci_allocate_bridge_windows() calls vmbus_allocate_mmio() to get a -32-bit MMIO range, it may get an MMIO range that overlaps with the +hv_allocate_config_window() calls vmbus_allocate_mmio() to get an +MMIO range, typically it gets a 32-bit MMIO range that overlaps with the framebuffer MMIO range, and later hv_pci_enter_d0() fails with an error message "PCI Pass-through VSP failed D0 Entry with status" since the host thinks that PCI devices must not use MMIO space that the @@ -31,7 +31,7 @@ Azure. I checked with the Hyper-V team and they said the statement should continue to be true for Gen2 VMs). In the first kernel, screen.lfb_base is not 0; if the user specifies a very high resolution, it's not enough -to only reserve 8MB: in this case, reserve half of the space below 4GB, +to only reserve 8MB: let's always reserve half of the space below 4GB, but cap the reservation to 128MB, which is the required framebuffer size of the highest resolution 7680*4320 supported by Hyper-V. @@ -42,7 +42,7 @@ Note: vmbus_reserve_fb() now also reserves an MMIO range at the beginning of the low MMIO range on CVMs, which have no framebuffers (the 'screen.lfb_base' in vmbus_reserve_fb() is 0 for CVMs), just in case the -host might treat the beginning of the low MMIO range specially [4]. BTW, +host might treat the beginning of the low MMIO range specially [3]. BTW, the OpenHCL kernel is not affected by the change, because that kernel boots with DeviceTree rather than ACPI (so vmbus_reserve_fb() won't run there), and there is no framebuffer device for that kernel. @@ -55,18 +55,20 @@ and the required framebuffer size exceeds 64MB (AFAIK, in practice, this isn't a typical configuration by users), the hyperv-drm driver may need to allocate an MMIO range above 4GB and change the framebuffer MMIO location -to the allocated MMIO range -- in this case, there can still be issues [3] +to the allocated MMIO range -- in this case, there can still be issues [4] which can't be easily fixed: any possible affected Gen1 users would have to use a resolution whose framebuffer size is <= 64MB, or switch to Gen2 VMs. [1] https://lore.kernel.org/all/sa1pr21mb692176c1bc53bfc9eae5cf8ebf...@sa1pr21mb6921.namprd21.prod.outlook.com/ [2] https://lore.kernel.org/all/sa1pr21mb69218f955b62dff62e3e88d2bf...@sa1pr21mb6921.namprd21.prod.outlook.com/ -[3] https://lore.kernel.org/all/sa1pr21mb69213486f821ca5a2c793c81bf...@sa1pr21mb6921.namprd21.prod.outlook.com/ -[4] https://lore.kernel.org/all/sn6pr02mb415726b17d5a6027cd1717e8d4...@sn6pr02mb4157.namprd02.prod.outlook.com/ +[3] https://lore.kernel.org/all/sn6pr02mb415726b17d5a6027cd1717e8d4...@sn6pr02mb4157.namprd02.prod.outlook.com/ +[4] https://lore.kernel.org/all/sa1pr21mb69213486f821ca5a2c793c81bf...@sa1pr21mb6921.namprd21.prod.outlook.com/ Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs") CC: [email protected] +Reviewed-by: Michael Kelley <[email protected]> +Tested-by: Krister Johansen <[email protected]> Signed-off-by: Dexuan Cui <[email protected]> --- @@ -104,6 +106,18 @@ Hi Hardik, I'm not adding your Reviewed-by since the patch changed. Please review the v2. + +Changes since v2: + Fixed the commit message: + hv_pci_allocate_bridge_windows() -> hv_allocate_config_window() + + Changed the "kdump" in the comment to "kdump/kexec or CVM" [Michael Kelley] + + Fixed the order of the "[3]" and "[4]" in the commit message. + + Added Krister's Tested-by. + Added Michael's Reviewed-by. + drivers/hv/vmbus_drv.c | 29 ++++++++++++++++++++++++++--- 1 file changed, 26 insertions(+), 3 deletions(-) @@ -141,8 +155,8 @@ + pr_warn("Unexpected low mmio base %pa\n", &low_mmio_base); + } else { + /* -+ * If the kdump kernel's lfb_base is 0, -+ * fall back to the low mmio base. ++ * If the kdump/kexec or CVM kernel's lfb_base ++ * is 0, fall back to the low mmio base. + */ + if (!start) + start = low_mmio_base; > Modulo my nit about the comment, > > Reviewed-by: Michael Kelley <[email protected]> Thanks a lot!
