There may be some differences in pci resource assignment between guest os
and firmware.

Eg. A Bridge with Bus [d2]
    -+-[0000:d2]---01.0-[d3]----01.0

    where [d2:01.00] is a pcie-pci-bridge with BAR0 (mem, 64-bit, non-pref) [size=256]
          [d3:01.00] is a PCI Device with BAR0 (mem, 64-bit, pref) [size=128K]
                                          BAR4 (mem, 64-bit, pref) [size=64M]

    In EDK2, the Resource Map would be:
        PciBus: Resource Map for Bridge [D2|01|00]
        Type = PMem64; Base = 0x8004000000;     Length = 0x4100000;     Alignment = 0x3FFFFFF
           Base = 0x8004000000; Length = 0x4000000;     Alignment = 0x3FFFFFF;  Owner = PCI [D3|01|00:20]
           Base = 0x8008000000; Length = 0x20000;       Alignment = 0x1FFFF;    Owner = PCI [D3|01|00:10]
        Type =  Mem64; Base = 0x8008100000;     Length = 0x100; Alignment = 0xFFF
    It would use 0x4100000 to calculate the root bus's PMem64 resource window.

    While in Linux, kernel will use 0x1FFFFFF as the alignment to calculate
    the PMem64 size, which would be 0x6000000. So kernel would try to
    allocate 0x6000000 from the PMem64 resource window, but since the window
    size is 0x4100000 as assigned by EDK2, the allocation would fail.

The diffences could result in resource assignment failure.

Using _DSM #5 method to inform guest os not to ignore the PCI configuration
that firmware has done at boot time could handle the differences.

Acked-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Jiahui Cen <cenjiahui@huawei.com>
Message-Id: <20210114100643.10617-5-cenjiahui@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>