Re: aarch64 efi boot failures with qemu 6.0+

[Ard Biesheuvel]

On Tue, 27 Jul 2021 at 07:12, Guenter Roeck <linux@roeck-us.net> 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.
>

EFI creates the mappings primarily for itself, and up until DSM #5
started to be enforced, all PCI resource allocations that existed at
boot were ignored by Linux and recreated from scratch.

Also, the commit in question looks dubious to me. I don't think it is
likely that Linux would fail to create a resource tree. What does
happen is that BARs get moved around, which may cause trouble in some
cases: for instance, we had to add special code to the EFI framebuffer
driver to copy with framebuffer BARs being relocated.

> 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.
>

I would argue that it is better to revert that commit. DSM #5 has a
long history of debate and misinterpretation, and while I think we
ended up with something sane, I don't think we should be using it in
this particular case.