Drivers: hv: Always select CONFIG_SYSFB for Hyper-V guests

The Hyper-V host provides guest VMs with a range of MMIO addresses
that guest VMBus drivers can use. The VMBus driver in Linux manages
that MMIO space, and allocates portions to drivers upon request. As
part of managing that MMIO space in a Generation 2 VM, the VMBus
driver must reserve the portion of the MMIO space that Hyper-V has
designated for the synthetic frame buffer, and not allocate this
space to VMBus drivers other than graphics framebuffer drivers. The
synthetic frame buffer MMIO area is described by the screen_info data
structure that is passed to the Linux kernel at boot time, so the
VMBus driver must access screen_info for Generation 2 VMs. (In
Generation 1 VMs, the framebuffer MMIO space is communicated to
the guest via a PCI pseudo-device, and access to screen_info is
not needed.)

In commit a07b50d8 ("hyperv: avoid dependency on screen_info")
the VMBus driver's access to screen_info is restricted to when
CONFIG_SYSFB is enabled. CONFIG_SYSFB is typically enabled in kernels
built for Hyper-V by virtue of having at least one of CONFIG_FB_EFI,
CONFIG_FB_VESA, or CONFIG_SYSFB_SIMPLEFB enabled, so the restriction
doesn't usually affect anything. But it's valid to have none of these
enabled, in which case CONFIG_SYSFB is not enabled, and the VMBus driver
is unable to properly reserve the framebuffer MMIO space for graphics
framebuffer drivers. The framebuffer MMIO space may be assigned to
some other VMBus driver, with undefined results. As an example, if
a VM is using a PCI pass-thru NVMe controller to host the OS disk,
the PCI NVMe controller is probed before any graphics devices, and the
NVMe controller is assigned a portion of the framebuffer MMIO space.
Hyper-V reports an error to Linux during the probe, and the OS disk
fails to get setup. Then Linux fails to boot in the VM.

Fix this by having CONFIG_HYPERV always select SYSFB. Then the
VMBus driver in a Gen 2 VM can always reserve the MMIO space for the
graphics framebuffer driver, and prevent the undefined behavior. But
don't select SYSFB when building for HYPERV_VTL_MODE as VTLs other
than VTL 0 don't have a framebuffer and aren't subject to the issue.
Adding SYSFB in such cases is harmless, but would increase the image
size for no purpose.

Fixes: a07b50d8 ("hyperv: avoid dependency on screen_info")
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Link: https://lore.kernel.org/stable/20250520040143.6964-1-mhklinux%40outlook.com
Link: https://lore.kernel.org/r/20250520040143.6964-1-mhklinux@outlook.com


Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20250520040143.6964-1-mhklinux@outlook.com>