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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.  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.

~Andrew

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