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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.04.18 at 14:01, <simon@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> 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)

Yes, there would be a problem: microcode_resume_cpu()
spin_lock()-s almost first thing, and this would break our
(simplistic) lock checking. Putting it also ahead of
spin_debug_enable() should work otoh.

Once at it, cpufreq_add_cpu() should be moved ahead of the
enable_cpu label as well, as cpufreq_del_cpu() wasn't called
yet at the point of the only goto to that label.

Jan


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