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

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

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.

Simon

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