Re: [Qemu-devel] [RFC] arm/cpu: fix soft lockup panic after resuming from stop

[Christoffer Dall]

Hi Steve,

On Wed, Mar 13, 2019 at 10:11:30AM +0000, Steven Price wrote:
> 
> Personally I think what we need is:
> 
> * Either a patch like the one from Heyi Guo (save/restore CNTVCT_EL0) or
> alternatively hooking up KVM_KVMCLOCK_CTRL to prevent the watchdog
> firing when user space explicitly stops scheduling the guest for a while.

If we save/restore CNTVCT_EL0 and the warning goes away, does the guest
wall clock timekeeping get all confused and does it figure this out
automagically somehow?

Does KVM_KVMCLOCK_CTRL solve that problem?

> 
> * KVM itself saving/restoring CNTVCT_EL0 during suspend/resume so the
> guest doesn't see time pass during a suspend.

This smells like policy to me so I'd much prefer keeping as much
functionality in user space as possible.  If we already have the APIs we
need from KVM, let's use them.

> 
> * Something equivalent to MSR_KVM_WALL_CLOCK_NEW for arm which allows
> the guest to query the wall clock time from the host and provides an
> offset between CNTVCT_EL0 to wall clock time which the KVM can update
> during suspend/resume. This means that during a suspend/resume the guest
> can observe that wall clock time has passed, without having to be
> bothered about CNTVCT_EL0 jumping forwards.
> 

Isn't the proper Arm architectural solution for this to read the
physical counter for wall clock time keeping ?

(Yes that will require a trap on physical counter reads after migration
on current systems, but migration sucks in terms of timekeeping
already.)


Thanks,

    Christoffer