Re: aarch64 efi boot failures with qemu 6.0+

[Michael S. Tsirkin]

On Mon, Jul 26, 2021 at 10:12:38PM -0700, Guenter Roeck wrote:
> 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.

I would say both. But about QEMU actually I think you have a point here.
Re-reading the spec:

0: No (The operating system shall not ignore the PCI configuration that 
firmware has done
at boot time. However, the operating system is free to configure the devices in 
this hierarchy
that have not been configured by the firmware. There may be a reduced level of 
hot plug
capability support in this hierarchy due to resource constraints. This 
situation is the same as
the legacy situation where this _DSM is not provided.)
1: Yes (The operating system may ignore the PCI configuration that the firmware 
has done
at boot time, and reconfigure/rebalance the resources in the hierarchy.)


I think I misread the spec previously, and understood it to mean that
1 means must ignore. In fact 1 gives the most flexibility.
So why are we suddenly telling the guest it must not override
firmware?

The commit log says
    The diffences could result in resource assignment failure.

which is kind of vague ...

Jiahui Cen, Igor, what do you think about it?
I'm inclined to revert 0cf8882fd06ba0aeb1e90fa6f23fce85504d7e14
at least for now.


> 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

Well it's not like we have an army of testers, I think we should
treat each problem report seriously ...

-- 
MST