From: Sasha Levin sashal@kernel.org

This is a note to let you know that I've just added the patch titled

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

to the 6.15-stable tree which can be found at:

https://git.kernel.org/pub/scm/linux/kernel/git/stable/stable-queue.git

The filename of the patch is: drivers-hv-always-select-config_sysfb-for-hyper-v-gu.patch and it can be found in the queue-6.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

Please DO NOT backport this patch to ANY stable trees, at least not at the moment. It is causing a config problem that we're trying to work out. Once the resolution is decided upon, we can figure out what to backport.

Thanks,

Michael Kelley

commit 9766859ee9884c35dde0411df167a06452fee3ce Author: Michael Kelley mhklinux@outlook.com Date: Mon May 19 21:01:43 2025 -0700

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

[ Upstream commit 96959283a58d91ae20d025546f00e16f0a555208 ]

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 a07b50d80ab6 ("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: a07b50d80ab6 ("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/st able%2F20250520040143.6964-1- mhklinux%2540outlook.com&data=05%7C02%7C%7C516ef64661c145eb315d08dda81861 51%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C638851544842065319%7CUn known%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXa W4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=FZnrM9REMICWp JqW88gD7CxeTwcztS2y8%2B8GqHNtF3E%3D&reserved=0 Link: https://lore.kernel.org/r%25 2F20250520040143.6964-1- mhklinux%40outlook.com&data=05%7C02%7C%7C516ef64661c145eb315d08dda8186151 %7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C638851544842081386%7CUnkn own%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4 zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=j%2F5AvOlxwTKfVUL vmNr%2FZliwRVrd9rSlVECyFGqFErE%3D&reserved=0 Signed-off-by: Wei Liu wei.liu@kernel.org Message-ID: 20250520040143.6964-1-mhklinux@outlook.com Signed-off-by: Sasha Levin sashal@kernel.org

diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig index 6c1416167bd2e..724fc08a73e70 100644 --- a/drivers/hv/Kconfig +++ b/drivers/hv/Kconfig @@ -9,6 +9,7 @@ config HYPERV select PARAVIRT select X86_HV_CALLBACK_VECTOR if X86 select OF_EARLY_FLATTREE if OF

• select SYSFB if !HYPERV_VTL_MODE
  help
    Select this option to run Linux as a Hyper-V client operating
    system.