Re: [PATCH] kvm: ppc: booke: Restore SPRG3 when entering guest

[Alexander Graf <agraf@suse.de>]

On 18.07.14 02:28, Scott Wood wrote:
> On Thu, 2014-07-17 at 18:29 +0200, Alexander Graf wrote:
>> On 17.07.14 18:27, Alexander Graf wrote:
>>> On 17.07.14 18:24, Bharat.Bhushan@freescale.com wrote:
>>>>> -----Original Message-----
>>>>> From: Alexander Graf [mailto:agraf@suse.de]
>>>>> Sent: Thursday, July 17, 2014 9:41 PM
>>>>> To: Bhushan Bharat-R65777; kvm-ppc@vger.kernel.org
>>>>> Cc: kvm@vger.kernel.org; Wood Scott-B07421; Yoder Stuart-B08248
>>>>> Subject: Re: [PATCH] kvm: ppc: booke: Restore SPRG3 when entering guest
>>>>>
>>>>>
>>>>> On 16.07.14 08:02, Bharat Bhushan wrote:
>>>>>> SPRG3 is guest accessible and SPRG3 can be clobbered by host or
>>>>>> another guest, So this need to be restored when loading guest state.
> SPRG3 is not guest writeable.  We should be doing this so that guest
> reads of SPRG3 through the alternative read-only SPR work, not because
> "SPRG3 can be clobbered by host or another guest".
>
>>>>>> Signed-off-by: Bharat Bhushan <Bharat.Bhushan@freescale.com>
>>>>>> ---
>>>>>>     arch/powerpc/kvm/booke_interrupts.S | 2 ++
>>>>>>     1 file changed, 2 insertions(+)
>>>>>>
>>>>>> diff --git a/arch/powerpc/kvm/booke_interrupts.S
>>>>>> b/arch/powerpc/kvm/booke_interrupts.S
>>>>>> index 2c6deb5ef..0d3403f 100644
>>>>>> --- a/arch/powerpc/kvm/booke_interrupts.S
>>>>>> +++ b/arch/powerpc/kvm/booke_interrupts.S
>>>>>> @@ -459,6 +459,8 @@ lightweight_exit:
>>>>>>          * written directly to the shared area, so we
>>>>>>          * need to reload them here with the guest's values.
>>>>>>          */
>>>>>> +    PPC_LD(r3, VCPU_SHARED_SPRG3, r5)
>>>>>> +    mtspr    SPRN_SPRG3, r3
>>>>> We also need to restore it when resuming the host, no?
>>>> I do not think host expect some meaningful value when returning from
>>>> guest, same true for SPRG4-7.
>>>> So there seems no reason to save host values and restore them.
> Linux no longer uses SPRG4-7 for itself.  That is not true of SPRG3, as
> Alex points out.
>
>>> Hmm - arch/powerpc/include/asm/reg.h says:
>>>
>>>   * All 32-bit:
>>>   *      - SPRG3 current thread_info pointer
>>>   *        (virtual on BookE, physical on others)
>>>
>>> but I can indeed find no trace of usage anywhere. This at least needs
>>> to go into the patch description.
>> Bah - it obviously is used. It's SPRN_SPRG_THREAD. And it's so
>> incredibly important that I have no idea how we could possibly run
>> without switching the host value back in very early. And even then our
>> interrupt handlers wouldn't work anymore.
>>
>> This is more complicated :).
> To make this work we need to avoid SPRG3 as well, or at least avoid
> using it for something needed prior to DO_KVM.
>
> We also need to update the documentation in reg.h to reflect the fact
> that we don't use SPRG4-7 anymore on e500.

I would personally prefer if we claim SPRG3R as unsupported on e500v2 
until we find someone who actually uses it. There's a good chance we'd 
start jumping through a lot of hoops and reduce overall performance for 
no real-world gain today.


Alex