Drivers: hv: vmbus: Don't release fb_mmio resource in vmbus_free_mmio()

The VMBus driver manages the MMIO space it owns via the hyperv_mmio
resource tree. Because the synthetic video framebuffer portion of the
MMIO space is initially setup by the Hyper-V host for each guest, the
VMBus driver does an early reserve of that portion of MMIO space in the
hyperv_mmio resource tree. It saves a pointer to that resource in
fb_mmio. When a VMBus driver requests MMIO space and passes "true"
for the "fb_overlap_ok" argument, the reserved framebuffer space is
used if possible. In that case it's not necessary to do another request
against the "shadow" hyperv_mmio resource tree because that resource
was already requested in the early reserve steps.

However, the vmbus_free_mmio() function currently does no special
handling for the fb_mmio resource. When a framebuffer device is
removed, or the driver is unbound, the current code for
vmbus_free_mmio() releases the reserved resource, leaving fb_mmio
pointing to memory that has been freed. If the same or another
driver is subsequently bound to the device, vmbus_allocate_mmio()
checks against fb_mmio, and potentially gets garbage. Furthermore
a second unbind operation produces this "nonexistent resource" error
because of the unbalanced behavior between vmbus_allocate_mmio() and
vmbus_free_mmio():

[   55.499643] resource: Trying to free nonexistent
			resource <0x00000000f0000000-0x00000000f07fffff>

Fix this by adding logic to vmbus_free_mmio() to recognize when
MMIO space in the fb_mmio reserved area would be released, and don't
release it. This filtering ensures the fb_mmio resource always exists,
and makes vmbus_free_mmio() more parallel with vmbus_allocate_mmio().

Fixes: be000f93 ("drivers:hv: Track allocations of children of hv_vmbus in private resource tree")
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Tested-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Reviewed-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250310035208.275764-1-mhklinux@outlook.com


Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20250310035208.275764-1-mhklinux@outlook.com>