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

[Alexander Graf <agraf@suse.de>]

On 18.07.14 02:36, Scott Wood wrote:
> On Fri, 2014-07-18 at 02:33 +0200, Alexander Graf wrote:
>> 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.
> The same problem applies to e500mc.

There we have SPRN_GSPRG3, no?


Alex