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

[Alexander Graf <agraf@suse.de>]

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.
>>>
>>> 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.

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.


Alex