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Re: [GIT PULL 0/1] EFI mixed mode fix for v4.18

On 11 July 2018 at 12:13, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> * Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>> The following changes since commit 1e4b044d22517cae7047c99038abb444423243ca:
>>   Linux 4.18-rc4 (2018-07-08 16:34:02 -0700)
>> are available in the Git repository at:
>>   git://git.kernel.org/pub/scm/linux/kernel/git/efi/efi.git tags/efi-urgent
>> for you to fetch changes up to d7f2e972e702d329fe11d6956df99dfc31211c25:
>>   efi/x86: remove pointless call to PciIo->Attributes() (2018-07-11 10:52:46 +0200)
>> ----------------------------------------------------------------
>> A single fix for the x86 PCI I/O protocol handling code that got
>> broken for mixed mode (64-bit Linux/x86 on 32-bit UEFI) after a
>> fix was applied in -rc2 to fix it for ordinary 64-bit Linux/x86.
> Just curious, because it's unclear from the changelog, what was the symptom, a
> boot hang, instant reboot, or some other misbehavior?

Hans reported that his mixed mode tablet would not boot at all any
more, but enter a reboot loop without any logs printed by the kernel.

> Also, what's the scope of
> the fix: were all 64-bit on 32-bit UEFI mixed-mode bootups affected, or only a
> certain subset?

Any mixed mode system with PCI is likely to be affected. I have added
a QEMU mixed mode config to my boot test environment to catch errors
like this one.

The unfortunate thing here is that this uncovered a fundamental issue
with mixed mode, i.e., that any UEFI protocol prototype involving
64-bit by-value parameters needs to be special cased in the stub code,
which is rather tedious. There is one other call that is potentially
affected, a file open call in the initrd handling code, but that
specific occurrence happens to work unmodified. This patch removes the
other one. Going forward, we will have to carefully review UEFI
protocol invocations for mixed mode compatibility.