Re: [Xen-devel] Resume from suspend to RAM broken when using early microcode updates

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

Andrew Cooper:
> On 11/04/18 12:48, Simon Gaiser wrote:
>> Hi,
>>
>> when I use early microcode loading with the microcode update with the
>> BTI mitigations, resuming from suspend to RAM is broken.
>>
>> Based on added logging to enter_state() (from power.c) it doesn't
>> survive the local_irq_restore(flags) call (at least a printk() after the
>> call doesn't output anything on the serial console).
>>
>> I guess that some irq handler tries to use IBRS/IBPB. But the microcode
>> is only loaded later.
>>
>> If I simply move the microcode_resume_cpu(0) directly before the
>> local_irq_restore(flags) everything seems to work fine. But I'm not sure
>> if this has unintended consequences.
>>
>> I tested the above with Xen 4.8.3 from Qubes which includes the BTI and
>> microcode patches from staging-4.8. AFAICS there are no commits which
>> changes the affected code or other commits which sound relevant so this
>> probably affected also all the newer branches.
> 
> S3 support is a very unloved area of the hypervisor.
> 
> Yes - we definitely need to get microcode reloaded before interrupts are
> enabled.

Do you see any problems with simply moving microcode_resume_cpu(0)
directly before the local_irq_restore(flags) call? (I'm not familiar
with the code at all and (early) resume handling sounds like something
which is easy to break in non obvious ways)

> That said, I would have expected a backtrace complaining about
> a GP fault if we had hit the use of IBRS/IBPB before the microcode was
> reloaded.

Yeah, not sure what's happening here. I don't get any output from after
local_irq_restore(flags). If you have some ideas for more debug output I
can easily test it.

Simon

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