Re: [PATCH] KVM: s390: force bp isolation for VSIE

[David Hildenbrand <david@redhat.com>]

On 14.02.2018 12:05, Christian Borntraeger wrote:
> 
> 
> On 02/14/2018 11:37 AM, David Hildenbrand wrote:
>> On 14.02.2018 11:14, Christian Borntraeger wrote:
>>>
>>>
>>> On 02/14/2018 11:06 AM, David Hildenbrand wrote:
>>>> On 14.02.2018 09:34, Christian Borntraeger wrote:
>>>>> If the guest runs with bp isolation when doing a SIE instruction,
>>>>> we must also run the nested guest with bp isolation when emulating
>>>>> that SIE instruction.
>>>>>
>>>>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>>>> ---
>>>>>  arch/s390/kvm/vsie.c | 15 +++++++++++++++
>>>>>  1 file changed, 15 insertions(+)
>>>>>
>>>>> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
>>>>> index ec772700ff96..b8e7660d7207 100644
>>>>> --- a/arch/s390/kvm/vsie.c
>>>>> +++ b/arch/s390/kvm/vsie.c
>>>>> @@ -821,6 +821,7 @@ static int do_vsie_run(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>>>>  {
>>>>>  	struct kvm_s390_sie_block *scb_s = &vsie_page->scb_s;
>>>>>  	struct kvm_s390_sie_block *scb_o = vsie_page->scb_o;
>>>>> +	int guest_bp_isolation;
>>>>>  	int rc;
>>>>>  
>>>>>  	handle_last_fault(vcpu, vsie_page);
>>>>> @@ -831,6 +832,15 @@ static int do_vsie_run(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>>>>  		s390_handle_mcck();
>>>>>  
>>>>>  	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>>>>> +
>>>>> +	/* save current guest state of bp isolation override */
>>>>> +	guest_bp_isolation = test_thread_flag(TIF_ISOLATE_BP_GUEST);
>>>>
>>>> If I am not wrong, this is not "guest state". The guest state is
>>>> vcpu->arch.sie_block->fpf . This is host state of a thread.
>>>
>>> Yes, this is the host thread that is going to "emulate" the vsie instruction
>>> by calling sie64a.
>>>
>>>>
>>>>> +
>>>>> +	/* if guest runs with bp isolation force it on nested guest */
>>>>> +	if (test_kvm_facility(vcpu->kvm, 82) &&
>>>>> +	    vcpu->arch.sie_block->fpf & FPF_BPBC)
>>>>> +		set_thread_flag(TIF_ISOLATE_BP_GUEST);
>>>>> +
>>>>>  	local_irq_disable();
>>>>>  	guest_enter_irqoff();
>>>>>  	local_irq_enable();
>>>>> @@ -840,6 +850,11 @@ static int do_vsie_run(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>>>>  	local_irq_disable();
>>>>>  	guest_exit_irqoff();
>>>>>  	local_irq_enable();
>>>>> +
>>>>> +	/* restore guest state for bp isolation override */
>>>>> +	if (!guest_bp_isolation)
>>>>> +		clear_thread_flag(TIF_ISOLATE_BP_GUEST);
>>>>> +
>>>>>  	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
>>>>>  
>>>>>  	if (rc == -EINTR) {
>>>>>
>>>>
>>>> You are trying to optimize the following case here:
>>>
>>> I am trying to fix a case where vsie would allow to disable branch prediction blocking.
>>>>
>>>> 1. TIF_ISOLATE_BP_GUEST is not set
>>>> 2. The guest has facility 82 and enabled FPF_BPBC
>>>
>>>
>>>> As the vSIE guest can change its FPF_BPBC, there is basically no
>>>> guarantee to that. So, when entering/leaving the nested guest, you act
>>>> like the hardware would be doing FPF_BPBC - as it could be disabled for
>>>> the nested guest / the nested guest can change the state itself.
>>>
>>> The BPBC is an effective control, so if you enter SIE with bp blocking,
>>> then the guest will have bp blocking "forced" on.
>>
>> The guest can at least disable BPBC logically. (you can enable the
>> control in the SCB but the guest can simply turn it off) - that's why we
>> sync it back in unshadow_scb().
>>
>> I now understand it like this:
>>
>> LPAR (BPBC = on) -> Guest BPBC value ignored
>> LPAR (BPBC = off) -> Guest BPBC value used
>> LPAR (BPBC = off) -> Guest (BPBC = off) -> Nested guest value used
>>
> 
> For full correctness:
> s/ignored/not relevant as the effective value is the logical OR/
> 
> but ignored is certainly good enough and shorter.
> 
>> And you are fixing this case:
>> LPAR (BPBC = off) -> Guest (BPBC = on) -> Nested guest ignored
> 
> 
> which would run the nested guest with BPBC off. 
> 
>>
>> And you do this by setting LPAR (BPBC = on) while running the nested guest.
> 
> yes.
> 
>>
>> If so, please add a comment
>>
>> /*
>>  * The guest is running with BPBC, so we have to force it on for our
>>  * nested guest. This is done by enabling BPBC globally, so the BPBC
>>  * control in the SCB (which the nested guest can modify) is simply
>>  * ignored.
>>  */
> 
> I will replace the
> /* if guest runs with bp isolation force it on nested guest */
> with your comment.

With that

Reviewed-by: David Hildenbrand <david@redhat.com>


-- 

Thanks,

David / dhildenb