> From: Michael Kelley <[email protected]> > Sent: Wednesday, April 29, 2026 11:01 AM > > From: Dexuan Cui <[email protected]> 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.
