On Tue, Dec 31, 2024 at 09:24:09PM +0000, Li, Yunxiang (Teddy) via SeaBIOS
wrote:
> [Public]
>
> Hi all,
>
> I've ran into an issue where the 64 bit address window allocated for one of
> the PCI-e host devices overlaps with a BIOS reserved range of
> fd00000000-ffffffffff, making that device unusable.
>
> 4000000000-7bfffffffff : PCI Bus 0000:00
> 4000000000-bfffffffff : PCI Bus 0000:0c
> 4000000000-7fffffffff : 0000:0c:00.0
> 8000000000-80001fffff : 0000:0c:00.0
> c000000000-13fffffffff : PCI Bus 0000:0b
> c000000000-ffffffffff : 0000:0b:00.0
> fd00000000-ffffffffff : Reserved
> 10000000000-100001fffff : 0000:0b:00.0
>
> 8000000000-7ffffffffff : PCI Bus 0000:00
> 8000000000-ffffffffff : PCI Bus 0000:0c
> 8000000000-bfffffffff : 0000:0c:00.0
> c000000000-c0001fffff : 0000:0c:00.0
> fd00000000-ffffffffff : Reserved
> 10000000000-17fffffffff : PCI Bus 0000:0b
> 10000000000-13fffffffff : 0000:0b:00.0
> 14000000000-140001fffff : 0000:0b:00.0
>
> I found that increasing the system ram of the VM to ~256G changes the address
> window and avoids the issue, and with some digging I think I found the root
> cause
>
> To begin, QEMU set this address range to be reserved here, and this is passed
> to Seabios via "etc/e820".
> https://gitlab.com/qemu-project/qemu/-/blob/master/hw/i386/pc.c#L865
> Separately QEMU pass to Seabios "etc/reserved-memory-end" here which is
> derived from machine->device_memory
> https://gitlab.com/qemu-project/qemu/-/blob/master/hw/i386/pc.c#L1007
>
> In Seabios, "etc/e820" is consumed here, which sets RamSizeOver4G only using
> the E820_RAM entries, ignoring any E820_RESERVED entries.
> https://gitlab.com/qemu-project/seabios/-/blob/master/src/fw/paravirt.c#L782
> Later "etc/reserved-memory-end" and RamSizeOver4G is used to determine the
> start of the PCI-e address window.
> https://gitlab.com/qemu-project/seabios/-/blob/master/src/fw/pciinit.c#L1138
>
> I think either QEMU should set etc/reserved-memory-end to be after both
> physical memory and the reserved ranges, or Seabios need to check both
> etc/820 and etc/reserved-memory-end. But I'm not sure which would be the
> correct move and indeed how to patch them.
commit 1e1da7a96300 ("check for e820 conflict") should fix that.
take care,
Gerd
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