Subject: Re: [PATCH 1/4] KVM: PPC: BOOK3S: PR: Emulate virtual timebase register

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Thu, 05 Jun 2014 23:03:15 +0530

Alexander Graf <[email-masked]> writes:

> On 05.06.14 17:50, Aneesh Kumar K.V wrote:
>> Alexander Graf <[email-masked]> writes:
>>
>>> On 05.06.14 14:08, Aneesh Kumar K.V wrote:
>>>> virtual time base register is a per VM, per cpu register that needs
>>>> to be saved and restored on vm exit and entry. Writing to VTB is not
>>>> allowed in the privileged mode.
>>>>
>>>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>

.......

>>>>    			break;
>>>> diff --git a/arch/powerpc/kvm/book3s_emulate.c b/arch/powerpc/kvm/book3s_emulate.c
>>>> index 3565e775b61b..1bb16a59dcbc 100644
>>>> --- a/arch/powerpc/kvm/book3s_emulate.c
>>>> +++ b/arch/powerpc/kvm/book3s_emulate.c
>>>> @@ -577,6 +577,9 @@ int kvmppc_core_emulate_mfspr_pr(struct kvm_vcpu *vcpu, int sprn, ulong *spr_val
>>>>    		 */
>>>>    		*spr_val = vcpu->arch.spurr;
>>>>    		break;
>>>> +	case SPRN_VTB:
>>>> +		*spr_val = vcpu->arch.vtb;
>>> Doesn't this mean that vtb can be the same 2 when the guest reads it 2
>>> times in a row without getting preempted?
>>
>> But a mfspr will result in VM exit and that would make sure we
>> update vcpu->arch.vtb with the correct value.
>
> We only call kvmppc_core_vcpu_put_pr() when we context switch away from 
> KVM, so it won't be updated, no?
>
>

kvmppc_copy_from_svcpu is also called from VM exit path (book3s_interrupt.S)

-aneesh