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

[Fabiano Rosas <farosas@linux.ibm.com>]

Nicholas Piggin <npiggin@gmail.com> 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 <farosas@linux.ibm.com>
>> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
>> ---
>>  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.

Or I could check for failure and clear vcpu->arch.emul_inst and
therefore also hr->heir if we couldn't load the instruction.

>> +
>> +		/* Don't leak the cause field */
>> +		hr->hfscr &= ~HFSCR_INTR_CAUSE;
>
> This hunk also remains -- shouldn't change HFSCR for HEA, only HFAC.

Ah of course, thanks.

>
> Thanks,
> Nick