Re: [PATCH] Drivers: hv: vmbus: Improve the logc of reserving fb_mmio on Gen2 VMs

[Krister Johansen]

On Thu, Apr 16, 2026 at 11:35:29AM -0700, Dexuan Cui wrote:
> If vmbus_reserve_fb() in the kdump kernel fails to properly reserve the
> framebuffer MMIO range 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.
> This is especially an issue if pci-hyperv is built-in and hyperv_drm is
> built as a module. Consequently, the kdump kernel fails to detect PCI
> devices via pci-hyperv, and may fail to mount the root file system,
> which may reside in a NVMe disk.
> 
> On Gen2 VMs, if the screen.lfb_base is 0 in the kdump kernel, fall
> back to the low MMIO base, which should be equal to the framebuffer
> MMIO base (Tested on x64 Windows Server 2016, and on x64 and ARM64 Windows
> Server 2025 and on Azure) [2]. In the first kernel, screen.lfb_base
> is not 0; if the user specifies a high resolution, it's not enough to
> only reserve 8MB: in this case, 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.
> 
> Add the cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) check, because a CoCo
> VM (i.e. Confidential VM) on Hyper-V doesn't have any framebuffer
> device, so there is no need to reserve any MMIO for it.
> 
> While at it, fix the comparison "end > VTPM_BASE_ADDRESS" by changing
> the > to >=. Here the 'end' is an inclusive end (typically, it's
> 0xFFFF_FFFF).
> 
> [1] https://lore.kernel.org/all/SA1PR21MB692176C1BC53BFC9EAE5CF8EBF51A@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/all/SA1PR21MB69218F955B62DFF62E3E88D2BF222@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> 
> Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs")
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> ---
>  drivers/hv/vmbus_drv.c | 30 ++++++++++++++++++++++++++++--
>  1 file changed, 28 insertions(+), 2 deletions(-)
 
Thanks for the updated patch.  I tested this on the arm64 instances that
had been failing and was able to confirm that without it present the
failure still occurred, but with the new patch networking was able to
attach correctly in the dump environment and kdumps were successful.

Tested-by: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>

-K