Re: [v1][PATCH 1/1] KVM: PPC: disable preemption when using hard_irq_disable()

["tiejun.chen" <tiejun.chen@windriver.com>]

On 07/14/2013 12:13 PM, Benjamin Herrenschmidt wrote:
> On Fri, 2013-07-12 at 12:54 +0800, tiejun.chen wrote:
>> Is the following fine?
>>
>> powerpc: to access local paca after hard irq disabled
>>
>> We can access paca directly after hard interrupt disabled, and
>> this can avoid accessing wrong paca when using get_paca() in
>> preempt case.
>>
>> Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>
>
> Ack. We still have an unresolved problem where gcc decides to copy r13
> to another register and then index from that, or even store and reload
> it, and this possibly accross preempt sections.
>
> It's unclear to me in what circumstances it will do it and whether
> there's a case of us getting completely screwed over, I need to
> investigate. This is the reason why we originally made the accesses to
> soft_enabled be inline asm.

Understood.

>
> We might need to do a bulk conversion of all PACA accesses to either
> such inline asm or "hide" r13 behind asm (forcing essentially a copy
> to another register on each use) or a combination of both.
>
> IE. inline asm for direct access of things like soft_enabled, and a
> get_paca/put_paca style interface that copies r13 and includes a
> preempt_disable/enable for the rest.
>

I'd like to check this possibility later.

Tiejun