Re: aarch64 efi boot failures with qemu 6.0+

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jul 27, 2021 at 09:04:20AM +0200, Ard Biesheuvel wrote:
> 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.

Re-reading it I have to agree. I think I misunderstood the spec and
guest behaviour when I applied it.

-- 
MST