Subject: Re: [PATCH] PPC: Don't sync timebase when inside VM

From: Scott Wood <[email-masked]>
Date: Fri, 2 Mar 2012 11:17:20 -0600

On 03/02/2012 10:30 AM, Alexander Graf wrote:
> 
> On 02.03.2012, at 17:20, Scott Wood wrote:
> 
>> On Fri, Mar 02, 2012 at 03:12:33PM +0100, Alexander Graf wrote:
>>> When running inside a virtual machine, we can not modify timebase, so
>>> let's just not call the functions for it then.
>>>
>>> This resolves hangs when booting e500 SMP guests on overcommitted hosts.
>>>
>>> Reported-by: Stuart Yoder <[email-masked]>
>>> Signed-off-by: Alexander Graf <[email-masked]>
>>> ---
>>> arch/powerpc/platforms/85xx/smp.c |    7 +++++++
>>> 1 files changed, 7 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/arch/powerpc/platforms/85xx/smp.c b/arch/powerpc/platforms/85xx/smp.c
>>> index ff42490..d4b6c1f 100644
>>> --- a/arch/powerpc/platforms/85xx/smp.c
>>> +++ b/arch/powerpc/platforms/85xx/smp.c
>>> @@ -249,6 +249,13 @@ void __init mpc85xx_smp_init(void)
>>> 		smp_85xx_ops.cause_ipi = doorbell_cause_ipi;
>>> 	}
>>>
>>> +	/* When running under a hypervisor, we can not modify tb */
>>> +	np = of_find_node_by_path("/hypervisor");
>>> +	if (np) {
>>> +		smp_85xx_ops.give_timebase = NULL;
>>> +		smp_85xx_ops.take_timebase = NULL;
>>> +	}
>>> +
>>> 	smp_ops = &smp_85xx_ops;
>>
>> Again, for 85xx we should *never* sync the timebase in the kernel,
>> hypervisor or no.
> 
> The code says "if the kexec config option is enabled, do the sync". I'm fairly sure it's there for a reason.

Sigh.  I forgot about that.  It's because instead of doing kexec the
simple way, we actually physically reset the core.  We really shouldn't
do that.  And we *really* shouldn't do it just because CONFIG_KEXEC is
defined, regardless of whether we're actually booting from kexec.

-Scott