Subject: Re: [PATCH v5 2/2] KVM: PPC: Book3S HV: Stop forwarding all HFUs to L1

From: Nicholas Piggin <[email-masked]>
Date: Thu, 29 Jul 2021 13:52:49 +1000

Excerpts from Fabiano Rosas's message of July 28, 2021 12:36 am:
> Nicholas Piggin <[email-masked]> writes:
> 
>> Excerpts from Fabiano Rosas's message of July 27, 2021 6:17 am:
>>> If the nested hypervisor has no access to a facility because it has
>>> been disabled by the host, it should also not be able to see the
>>> Hypervisor Facility Unavailable that arises from one of its guests
>>> trying to access the facility.
>>> 
>>> This patch turns a HFU that happened in L2 into a Hypervisor Emulation
>>> Assistance interrupt and forwards it to L1 for handling. The ones that
>>> happened because L1 explicitly disabled the facility for L2 are still
>>> let through, along with the corresponding Cause bits in the HFSCR.
>>> 
>>> Signed-off-by: Fabiano Rosas <[email-masked]>
>>> Reviewed-by: Nicholas Piggin <[email-masked]>
>>> ---
>>>  arch/powerpc/kvm/book3s_hv_nested.c | 32 +++++++++++++++++++++++------
>>>  1 file changed, 26 insertions(+), 6 deletions(-)
>>> 
>>> diff --git a/arch/powerpc/kvm/book3s_hv_nested.c b/arch/powerpc/kvm/book3s_hv_nested.c
>>> index 8215dbd4be9a..d544b092b49a 100644
>>> --- a/arch/powerpc/kvm/book3s_hv_nested.c
>>> +++ b/arch/powerpc/kvm/book3s_hv_nested.c
>>> @@ -99,7 +99,7 @@ static void byteswap_hv_regs(struct hv_guest_state *hr)
>>>  	hr->dawrx1 = swab64(hr->dawrx1);
>>>  }
>>>  
>>> -static void save_hv_return_state(struct kvm_vcpu *vcpu, int trap,
>>> +static void save_hv_return_state(struct kvm_vcpu *vcpu,
>>>  				 struct hv_guest_state *hr)
>>>  {
>>>  	struct kvmppc_vcore *vc = vcpu->arch.vcore;
>>> @@ -118,7 +118,7 @@ static void save_hv_return_state(struct kvm_vcpu *vcpu, int trap,
>>>  	hr->pidr = vcpu->arch.pid;
>>>  	hr->cfar = vcpu->arch.cfar;
>>>  	hr->ppr = vcpu->arch.ppr;
>>> -	switch (trap) {
>>> +	switch (vcpu->arch.trap) {
>>>  	case BOOK3S_INTERRUPT_H_DATA_STORAGE:
>>>  		hr->hdar = vcpu->arch.fault_dar;
>>>  		hr->hdsisr = vcpu->arch.fault_dsisr;
>>> @@ -128,9 +128,29 @@ static void save_hv_return_state(struct kvm_vcpu *vcpu, int trap,
>>>  		hr->asdr = vcpu->arch.fault_gpa;
>>>  		break;
>>>  	case BOOK3S_INTERRUPT_H_FAC_UNAVAIL:
>>> -		hr->hfscr = ((~HFSCR_INTR_CAUSE & hr->hfscr) |
>>> -			     (HFSCR_INTR_CAUSE & vcpu->arch.hfscr));
>>> -		break;
>>> +	{
>>> +		u8 cause = vcpu->arch.hfscr >> 56;
>>
>> Can this be u64 just to help gcc?
>>
> 
> Yes.
> 
>>> +
>>> +		WARN_ON_ONCE(cause >= BITS_PER_LONG);
>>> +
>>> +		if (!(hr->hfscr & (1UL << cause))) {
>>> +			hr->hfscr = ((~HFSCR_INTR_CAUSE & hr->hfscr) |
>>> +				     (HFSCR_INTR_CAUSE & vcpu->arch.hfscr));
>>> +			break;
>>> +		}
>>> +
>>> +		/*
>>> +		 * We have disabled this facility, so it does not
>>> +		 * exist from L1's perspective. Turn it into a HEAI.
>>> +		 */
>>> +		vcpu->arch.trap = BOOK3S_INTERRUPT_H_EMUL_ASSIST;
>>> +		kvmppc_load_last_inst(vcpu, INST_GENERIC, &vcpu->arch.emul_inst);
>>
>> Hmm, this doesn't handle kvmpc_load_last_inst failure. Other code tends 
>> to just resume guest and retry in this case. Can we do that here?
>>
> 
> Not at this point. The other code does that inside
> kvmppc_handle_exit_hv, which is called from kvmhv_run_single_vcpu. And
> since we're changing the interrupt, I cannot load the last instruction
> at kvmppc_handle_nested_exit because at that point this is still an HFU.
> 
> Unless I do it anyway at the HFU handler and put a comment explaining
> the situation.

Yeah I think it would be better to move this logic to the nested exit 
handler.

Thanks,
Nick