[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: aarch64 efi boot failures with qemu 6.0+

From: Guenter Roeck
Subject: Re: aarch64 efi boot failures with qemu 6.0+
Date: Wed, 28 Jul 2021 07:03:35 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

On 7/28/21 6:25 AM, Ard Biesheuvel wrote:
On Wed, 28 Jul 2021 at 15:11, Michael S. Tsirkin <mst@redhat.com> wrote:

On Tue, Jul 27, 2021 at 12:36:03PM +0200, Igor Mammedov wrote:
On Tue, 27 Jul 2021 05:01:23 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

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
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

... 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

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

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.
Looking at patch history, it seems consensus was that it's better to
enforce firmware allocations.

Also letting OS do as it pleases might break PCI devices that
don't tolerate reallocation. ex: firmware initializes PCI device
IO/BARs and then fetches ACPI tables, which get patched with
assigned resources.

to me returning 0 seems to be correct choice.
In addition resource hinting also works via firmware allocations,
if we revert the commit it might change those configs.

Well if firmware people now tell us their allocations were never
intended for guest OS use then maybe we should not intervene.

DSM #5 was introduced to permit firmware running on x86_64 systems to
boot 32-bit OSes (read Windows) unmodified, while still leaving
enlightened, 64-bit OSes the opportunity to reorganize the BARs if
there is sufficient space in the resource windows, and if the OS runs
in long mode so it can address all of it.

This is why the default-if-absent according to the spec is '0', and I
already explained up-thread why arm64 deviates from this.

But Igor has a point: there are cases where especially bus numbers
should not be touched, as firmware tables consumed by the OS may carry
b/d/f identifiers for things like SMMU pass through, where changing
the bus numbers obviously invalidates this information.

These are exceptional cases, though, and I would argue that these
should be considered individually, rather than setting DSM #5 to 0x0
simply because there might be cases where not doing so could
theoretically break things, given that doing so has proven to break

As others noted the original commit was kind of vague:

1. it said "Using _DSM #5 method to inform guest os not to ignore the PCI 
that firmware has done at boot time could handle the differences."
which is not what the spec says and not what the patch did -
guest os does not ignore configuration even without this,
it is just allowed to change it.

2. is says could result but does not report whether that happened in the

Given this causes a regression I'm inclined to just revert for now.
We can figure it out for the next release.

For a revert of commit 0cf8882fd06ba0aeb1e90fa6f23fce85504d7e14, feel
free to include

Acked-by: Ard Biesheuvel <ardb@kernel.org>


Tested-by: Guenter Roeck <linux@roeck-us.net>

and please also involve me if any future debates on this subject flare up again.

Same here.


reply via email to

[Prev in Thread] Current Thread [Next in Thread]