[Bug 254956] Assertion fails when passing-through PCI device with large memory BAR

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254956

            Bug ID: 254956
           Summary: Assertion fails when passing-through PCI device with
                    large memory BAR
           Product: Base System
           Version: 13.0-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bhyve
          Assignee: virtualization at FreeBSD.org
          Reporter: crowston at protonmail.com

Passing-through a PCI device with a large BAR (for instance, a GPU) fails when
the 0xfff... probe address is assigned to the BAR: bhyve exists with an
assertion failed such as

Assertion failed: (error == 0), function modify_bar_registration, file
/usr/src/usr.sbin/bhyve/pci_emul.c, line 501.

During boot the system assigns the BAR a temporary address such that addr +
size = 0xffffffff. It immediately restores the previous address. This is not a
problem for most PCI devices. However, when passing through a GPU, the
temporary memory region is large enough to occupy space already taken by the
high performance time counter. This causes the assertion to fail at run time.

I propose that when a probing address such as this is assigned to the BAR, we
do not attempt to intercept it at all. My proposal is here:
https://reviews.freebsd.org/D29698

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