On Sat, May 17, 2025 at 06:47:22PM +0000, Michael Kelley wrote:
> From: Saurabh Singh Sengar <ssen...@linux.microsoft.com> Sent: Saturday, May
> 17, 2025 9:14 AM
> >
> > On Sat, May 17, 2025 at 01:34:20PM +0000, Michael Kelley wrote:
> > > From: Saurabh Singh Sengar <ssen...@microsoft.com> Sent: Friday, May 16,
> > > 2025 9:38 PM
> > > >
> > > > > From: Michael Kelley <mhkli...@outlook.com>
> > > > >
> > > > > 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
> > > > > graphic 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.
> > > >
> > > > One question: Shouldn't the SYSFB be selected by actual graphics
> > > > framebuffer driver
> > > > which is expected to use it. With this patch this option will be
> > > > enabled irrespective
> > > > if there is any user for it or not, wondering if we can better optimize
> > > > it for such systems.
> > > >
> > >
> > > That approach doesn't work. For a cloud-based server, it might make
> > > sense to build a kernel image without either of the Hyper-V graphics
> > > framebuffer drivers (DRM_HYPERV or HYPERV_FB) since in that case the
> > > Linux console is the serial console. But the problem could still occur
> > > where a PCI pass-thru NVMe controller tries to use the MMIO space
> > > that Hyper-V intends for the framebuffer. That problem is directly tied
> > > to CONFIG_SYSFB because it's the VMBus driver that must treat the
> > > framebuffer MMIO space as special. The absence or presence of a
> > > framebuffer driver isn't the key factor, though we've been (incorrectly)
> > > relying on the presence of a framebuffer driver to set CONFIG_SYSFB.
> > >
> >
> > Thank you for the clarification. I was concerned because SYSFB is not
> > currently
> > enabled in the OpenHCL kernel, and our goal is to keep the OpenHCL
> > configuration
> > as minimal as possible. I haven't yet looked into the details to determine
> > whether this might have any impact on the kernel binary size or runtime
> > memory
> > usage. I trust this won't affect negatively.
> >
> > OpenHCL Config Ref:
> > https://github.com/microsoft/OHCL-Linux-Kernel/blob/product/hcl-main/6.12/Microsoft/hcl-x64.config
> >
>
> Good point.
>
> The OpenHCL code tree has commit a07b50d80ab6 that restricts the
> screen_info to being available only when CONFIG_SYSFB is enabled.
> But since OpenHCL in VTL2 gets its firmware info via OF instead of ACPI,
> I'm unsure what the Hyper-V host tells it about available MMIO space,
> and whether that space includes MMIO space for a framebuffer. If it
> doesn't, then OpenHCL won't have the problem I describe above, and
> it won't need CONFIG_SYSFB. This patch could be modified to do
>
> select SYSFB if !HYPERV_VTL_MODE
I am worried that this is not very scalable, there could be more such
Hyper-V systems in future.
>
> Can you find out what MMIO space Hyper-V provides to VTL2 via OF?
> It would make sense if no framebuffer is provided. And maybe
> screen_info itself is not set up when VTL2 is loaded, which would
> also make adding CONFIG_SYSFB pointless for VTL2.
I can only see below address range passed for MMIO to VMBus driver:
ranges = <0x0f 0xf0000000 0x0f 0xf0000000 0x10000000>;
I don't think we have any use of scrren_info or framebuffer in OpenHCL.
- Saurabh
