Message-ID:
 <SN6PR02MB41574848C3351DD1673A2855D492A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Sat, 17 May 2025 18:47:22 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
CC: Saurabh Singh Sengar <ssengar@...rosoft.com>, KY Srinivasan
	<kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	"deller@....de" <deller@....de>, "javierm@...hat.com" <javierm@...hat.com>,
	"arnd@...db.de" <arnd@...db.de>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "stable@...r.kernel.org"
	<stable@...r.kernel.org>
Subject: RE: [EXTERNAL] [PATCH 1/1] Drivers: hv: Always select CONFIG_SYSFB
 for Hyper-V guests

From: Saurabh Singh Sengar <ssengar@...ux.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 <ssengar@...rosoft.com> Sent: Friday, May 16, 2025 9:38 PM
> > >
> > > > From: Michael Kelley <mhklinux@...look.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

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.

Michael

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