[PATCH] KVM: PPC: Don't sync timebase when inside KVM

Alexander Graf agraf at suse.de
Thu Mar 1 05:28:20 EST 2012

On 29.02.2012, at 18:50, Scott Wood <scottwood at freescale.com> wrote:

> On 02/28/2012 08:16 PM, Alexander Graf wrote:
>> When we know that we're running inside of a KVM guest, we don't have to
>> worry about synchronizing timebases between different CPUs, since the
>> host already took care of that.
>> This fixes CPU overcommit scenarios where vCPUs could hang forever trying
>> to sync each other while not being scheduled.
>> Reported-by: Stuart Yoder <B08248 at freescale.com>
>> Signed-off-by: Alexander Graf <agraf at suse.de>
> This should apply to any hypervisor, not just KVM.  

Sure, but do you have a generic function to evaluate that? :)

> On book3e, Power ISA
> says timebase is read-only on virtualized implementations.  My
> understanding is that book3s is paravirt-only (guest state is not
> considered an implementation of the Power ISA), and it says "Writing the
> Time Base is privileged, and can be done only in hypervisor state".

For PR non-PAPR KVM, we are non-paravirt, but ignore tb writes iirc.

> Which platforms are you seeing this on?  If it's on Freescale chips,
> U-Boot should be doing the sync and Linux should never do it, even in
> the absence of a hypervisor.

This is on e500mc.


> -Scott
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