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

[Dexuan Cui]

> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Sent: Wednesday, April 29, 2026 11:01 AM
> 
> From: Dexuan Cui <DECUI@xxxxxxxxxxxxx> Sent: Tuesday, April 28, 2026 8:13
> PM
>  ...
> >
> > A CVM on Hyper-V won't start without the command line
> >     Disable-VMConsoleSupport -VMName $vmName
This is not true. It turns out I can start a VBS/SNP/TDX without the
command line.... Sorry! Not sure why I had the wrong impression -- I
guess I was told to always run the command since day 1, so I subconsciously
thought a VM would not start without it. Or, maybe the host behavior
changed? but that seems unlikely to me.

> Unfortunately, on my laptop Hyper-V, a VM with VBS Isolation appears
> to *not* require Disable-VMConsoleSupport. I can start the VM, and the
> VM is offered the VMBus synthvid, mouse, and keyboard devices.
Actually I can also start a VBS VM without Disable-VMConsoleSupport.

> But what's weird in this case is that vmbus_reserved_fb() sees lfb_base
> and lfb_start as 0. 
I see the same.

> Furthermore, as a test, I changed the "allowed_in_isolated"
> flag to true for the synthvid device, and the Hyper-V DRM driver loads and
> initializes. 
I also changed the flag .allowed_in_isolated to true for HV_SYNTHVID_GUID,
HV_KBD, and HV_MOUSE, but I can't see the devices in "lsvmbus".

In vmbus_onoffer(), I printed the offer->offer.if_type and
offer->offer.if_instance just after the message " Invalid offer %d from the host
supporting isolation", and I indeed don't see the fb/mouse/keyboard devices.

I'm on a recent Hyper-V dev build. Maybe this is why my observation is
not exactly the same.

>In doing so, the vmconnect.exe window is resized larger, as is
> done in a normal VM. /proc/iomem shows that the DRM driver claimed
> the expected MMIO range at the start of low MMIO space. I can run a user
> space program that mmaps /dev/fb0 and writes pixels to the mmap'ed
> memory, and that succeeds as it would in a normal VM, but the
> vmconnect.exe window doesn't show anything. It appears that the Hyper-V
> host has allocated memory for the frame buffer, but is ignoring anything
> that is written to it.
> 
> Running Disable-VMConsoleSupport works as expected -- the synthvid,
> mouse, and keyboard devices are no longer offered to the VM.

I even ran "Enable-VMConsoleSupport", which finished without any error,
but I still didn't see the keyboard/mouse/framebuffer devices.
 
> So instead of not reserving any MMIO space for the framebuffer on
> CVMs, the code you already have limits the reservation to half of the
> MMIO space below 4 GB. 
Correct.

> Won't that work to avoid exhausting the low
> MMIO space in a CVM that's running on a local Hyper-V with only 128
> MiB of low MMIO space?
Correct. I'll drop the CVM check in vmbus_reserve_fb() in v2.