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Date: Thu, 01 Jul 2021 18:04:34 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [RFC PATCH 38/43] KVM: PPC: Book3S HV P9: Test dawr_enabled()
 before saving host DAWR SPRs
To: Fabiano Rosas <farosas@linux.ibm.com>, kvm-ppc@vger.kernel.org
References: <20210622105736.633352-1-npiggin@gmail.com>
 <20210622105736.633352-39-npiggin@gmail.com> <87eecj2qcv.fsf@linux.ibm.com>
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Excerpts from Fabiano Rosas's message of July 1, 2021 3:51 am:
> Nicholas Piggin <npiggin@gmail.com> writes:
>=20
>> Some of the DAWR SPR access is already predicated on dawr_enabled(),
>> apply this to the remainder of the accesses.
>>
>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>> ---
>>  arch/powerpc/kvm/book3s_hv_p9_entry.c | 34 ++++++++++++++++-----------
>>  1 file changed, 20 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/powerpc/kvm/book3s_hv_p9_entry.c b/arch/powerpc/kvm/bo=
ok3s_hv_p9_entry.c
>> index 7aa72efcac6c..f305d1d6445c 100644
>> --- a/arch/powerpc/kvm/book3s_hv_p9_entry.c
>> +++ b/arch/powerpc/kvm/book3s_hv_p9_entry.c
>> @@ -638,13 +638,16 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64=
 time_limit, unsigned long lpc
>>
>>  	host_hfscr =3D mfspr(SPRN_HFSCR);
>>  	host_ciabr =3D mfspr(SPRN_CIABR);
>> -	host_dawr0 =3D mfspr(SPRN_DAWR0);
>> -	host_dawrx0 =3D mfspr(SPRN_DAWRX0);
>>  	host_psscr =3D mfspr(SPRN_PSSCR);
>>  	host_pidr =3D mfspr(SPRN_PID);
>> -	if (cpu_has_feature(CPU_FTR_DAWR1)) {
>> -		host_dawr1 =3D mfspr(SPRN_DAWR1);
>> -		host_dawrx1 =3D mfspr(SPRN_DAWRX1);
>> +
>> +	if (dawr_enabled()) {
>> +		host_dawr0 =3D mfspr(SPRN_DAWR0);
>> +		host_dawrx0 =3D mfspr(SPRN_DAWRX0);
>> +		if (cpu_has_feature(CPU_FTR_DAWR1)) {
>> +			host_dawr1 =3D mfspr(SPRN_DAWR1);
>> +			host_dawrx1 =3D mfspr(SPRN_DAWRX1);
>=20
> The userspace needs to enable DAWR1 via KVM_CAP_PPC_DAWR1. That cap is
> not even implemented in QEMU currently, so we never allow the guest to
> set vcpu->arch.dawr1. If we check for kvm->arch.dawr1_enabled instead of
> the CPU feature, we could shave some more time here.

Ah good point, yes let's do that.

Thanks,
Nick