Re: aarch64 efi boot failures with qemu 6.0+

[Guenter Roeck <linux@roeck-us.net>]

On 7/26/21 9:45 PM, Michael S. Tsirkin wrote:
> On Mon, Jul 26, 2021 at 06:00:57PM +0200, Ard Biesheuvel wrote:
>> (cc Bjorn)
>>
>> On Mon, 26 Jul 2021 at 11:08, Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
>>>
>>> On 7/26/21 12:56 AM, Guenter Roeck wrote:
>>>> On 7/25/21 3:14 PM, Michael S. Tsirkin wrote:
>>>>> On Sat, Jul 24, 2021 at 11:52:34AM -0700, Guenter Roeck wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> starting with qemu v6.0, some of my aarch64 efi boot tests no longer
>>>>>> work. Analysis shows that PCI devices with IO ports do not instantiate
>>>>>> in qemu v6.0 (or v6.1-rc0) when booting through efi. The problem affects
>>>>>> (at least) ne2k_pci, tulip, dc390, and am53c974. The problem only
>>>>>> affects
>>>>>> aarch64, not x86/x86_64.
>>>>>>
>>>>>> I bisected the problem to commit 0cf8882fd0 ("acpi/gpex: Inform os to
>>>>>> keep firmware resource map"). Since this commit, PCI device BAR
>>>>>> allocation has changed. Taking tulip as example, the kernel reports
>>>>>> the following PCI bar assignments when running qemu v5.2.
>>>>>>
>>>>>> [    3.921801] pci 0000:00:01.0: [1011:0019] type 00 class 0x020000
>>>>>> [    3.922207] pci 0000:00:01.0: reg 0x10: [io  0x0000-0x007f]
>>>>>> [    3.922505] pci 0000:00:01.0: reg 0x14: [mem 0x10000000-0x1000007f]
>>
>> IIUC, these lines are read back from the BARs
>>
>>>>>> [    3.927111] pci 0000:00:01.0: BAR 0: assigned [io  0x1000-0x107f]
>>>>>> [    3.927455] pci 0000:00:01.0: BAR 1: assigned [mem
>>>>>> 0x10000000-0x1000007f]
>>>>>>
>>
>> ... and this is the assignment created by the kernel.
>>
>>>>>> With qemu v6.0, the assignment is reported as follows.
>>>>>>
>>>>>> [    3.922887] pci 0000:00:01.0: [1011:0019] type 00 class 0x020000
>>>>>> [    3.923278] pci 0000:00:01.0: reg 0x10: [io  0x0000-0x007f]
>>>>>> [    3.923451] pci 0000:00:01.0: reg 0x14: [mem 0x10000000-0x1000007f]
>>>>>>
>>
>> The problem here is that Linux, for legacy reasons, does not support
>> I/O ports <= 0x1000 on PCI, so the I/O assignment created by EFI is
>> rejected.
>>
>> This might make sense on x86, where legacy I/O ports may exist, but on
>> other architectures, this makes no sense.
> 
> 
> Fixing Linux makes sense but OTOH EFI probably shouldn't create mappings
> that trip up existing guests, right?
> 

I think it is difficult to draw a line. Sure, maybe EFI should not create
such mappings, but then maybe qemu should not suddenly start to enforce
those mappings for existing guests either.

For my own testing, I simply reverted commit 0cf8882fd0 in my copy of
qemu. That solves my immediate problem, giving us time to find a solution
that is acceptable for everyone. After all, it doesn't look like anyone
else has noticed the problem, so there is no real urgency.

Thanks,
Guenter