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

[Hardik Garg <hargar@linux.microsoft.com>]

On 4/16/2026 11:35 AM, 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@SA1PR21MB6921.namprd21.prod.outlook.com/
> [2] https://lore.kernel.org/all/SA1PR21MB69218F955B62DFF62E3E88D2BF222@SA1PR21MB6921.namprd21.prod.outlook.com/
> 
> Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs")
> CC: stable@vger.kernel.org
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
>  drivers/hv/vmbus_drv.c | 30 ++++++++++++++++++++++++++++--
>  1 file changed, 28 insertions(+), 2 deletions(-)
Reviewed-by: Hardik Garg <hargar@linux.microsoft.com>




Thanks,
Hardik