Re: [Qemu-devel] [PATCH 24/58] PPC: E500: Add PV spinning code

[Alexander Graf]

Am 17.09.2011 um 18:58 schrieb Blue Swirl <address@hidden>:

> On Wed, Sep 14, 2011 at 8:42 AM, Alexander Graf <address@hidden> wrote:
>> CPUs that are not the boot CPU need to run in spinning code to check if they
>> should run off to execute and if so where to jump to. This usually happens
>> by leaving secondary CPUs looping and checking if some variable in memory
>> changed.
>> 
>> In an environment like Qemu however we can be more clever. We can just export
>> the spin table the primary CPU modifies as MMIO region that would event based
>> wake up the respective secondary CPUs. That saves us quite some cycles while
>> the secondary CPUs are not up yet.
>> 
>> So this patch adds a PV device that simply exports the spinning table into 
>> the
>> guest and thus allows the primary CPU to wake up secondary ones.
> 
> On Sparc32, there is no need for a PV device. The CPU is woken up from
> halted state with an IPI. Maybe you could use this approach?

The way it's done here is defined by u-boot and now also nailed down in the 
ePAPR architecture spec. While alternatives might be more appealing, this is 
how guests work today :).

Alex

>