Re: [PATCH 01/12] KVM: PPC: Book3S HV nestedv2: Invalidate RPT before deleting a guest

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Vaibhav Jain <vaibhav@linux.ibm.com> writes:

> Hi Aneesh,
>
> Thanks for looking into this patch. My responses inline below:
>
> "Aneesh Kumar K.V (IBM)" <aneesh.kumar@kernel.org> writes:
>
>> Vaibhav Jain <vaibhav@linux.ibm.com> writes:
>>
>>> From: Jordan Niethe <jniethe5@gmail.com>
>>>
>>> An L0 must invalidate the L2's RPT during H_GUEST_DELETE if this has not
>>> already been done. This is a slow operation that means H_GUEST_DELETE
>>> must return H_BUSY multiple times before completing. Invalidating the
>>> tables before deleting the guest so there is less work for the L0 to do.
>>>
>>> Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
>>> ---
>>>  arch/powerpc/include/asm/kvm_book3s.h | 1 +
>>>  arch/powerpc/kvm/book3s_hv.c          | 6 ++++--
>>>  arch/powerpc/kvm/book3s_hv_nested.c   | 2 +-
>>>  3 files changed, 6 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
>>> index 4f527d09c92b..a37736ed3728 100644
>>> --- a/arch/powerpc/include/asm/kvm_book3s.h
>>> +++ b/arch/powerpc/include/asm/kvm_book3s.h
>>> @@ -302,6 +302,7 @@ void kvmhv_nested_exit(void);
>>>  void kvmhv_vm_nested_init(struct kvm *kvm);
>>>  long kvmhv_set_partition_table(struct kvm_vcpu *vcpu);
>>>  long kvmhv_copy_tofrom_guest_nested(struct kvm_vcpu *vcpu);
>>> +void kvmhv_flush_lpid(u64 lpid);
>>>  void kvmhv_set_ptbl_entry(u64 lpid, u64 dw0, u64 dw1);
>>>  void kvmhv_release_all_nested(struct kvm *kvm);
>>>  long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu);
>>> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
>>> index 1ed6ec140701..5543e8490cd9 100644
>>> --- a/arch/powerpc/kvm/book3s_hv.c
>>> +++ b/arch/powerpc/kvm/book3s_hv.c
>>> @@ -5691,10 +5691,12 @@ static void kvmppc_core_destroy_vm_hv(struct kvm *kvm)
>>>  			kvmhv_set_ptbl_entry(kvm->arch.lpid, 0, 0);
>>>  	}
>>>  
>>> -	if (kvmhv_is_nestedv2())
>>> +	if (kvmhv_is_nestedv2()) {
>>> +		kvmhv_flush_lpid(kvm->arch.lpid);
>>>  		plpar_guest_delete(0, kvm->arch.lpid);
>>>
>>
>> I am not sure I follow the optimization here. I would expect the
>> hypervisor to kill all the translation caches as part of guest_delete.
>> What is the benefit of doing a lpid flush outside the guest delete?
>>
> Thats right. However without this optimization the H_GUEST_DELETE hcall
> in plpar_guest_delete() returns H_BUSY multiple times resulting in
> multiple hcalls to the hypervisor until it finishes. Flushing the guest
> translation cache upfront reduces the number of HCALLs L1 guests has to
> make to delete a L2 guest via H_GUEST_DELETE.
>

can we add that as a comment above that kvmhv_flush_lpid()?

-aneesh