[Bug 222996] FreeBSD 11.1-12 on Hyper-V with PCI Express Pass Through

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222996

--- Comment #36 from Wei Hu <weh at microsoft.com> ---
(In reply to Dexuan Cui from comment #35)
On Gen1, the system provided pcib driver grabs these mmio addresses way earlier
than vmbus comes up. I then allocates the address to the vga driver:

pcib0: allocated type 3 (0xf8000000-0xfbffffff) for rid 10 of pci0:0:8:0

This happened before vmbus comes up.

On Gen2 I just look at my test VM without having a passthrough deivce. There is
no pcib device comes before vmbus loading. The efifb (framebuffer) driver is
loaded before vmbus but it doesn't show which mmio address it is using. When
vmbus comes up it gets the address:
vmbus0: decoding 3 range 0xf8000000-0xfed3ffff

I am not sure whether the passthrough device comes before or after vmbus is
loaded. If it comes before, then this patch may not work. We may need to
reserve the address before the passthrough device asks for mmio address. Once
Dmitry has the environment we can find out.

The bad address we know now is just 8MB (0xf8000000 - 0xf87fffff). I think the
default maximum space HyperV gives for frame buffer is 64MB which should be ok.

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