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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.04.18 at 14:11, <JBeulich@xxxxxxxx> wrote:
>>>> 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.

And I think console_end_sync() want to be moved earlier then
as well.

Jan


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