Messages in this thread | | | Subject | Re: [PATCH v1 1/2] s390x: KVM: accept STSI for CPU topology information | From | Pierre Morel <> | Date | Thu, 15 Jul 2021 13:37:04 +0200 |
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On 7/15/21 12:19 PM, David Hildenbrand wrote: > On 15.07.21 12:16, Cornelia Huck wrote: >> On Thu, Jul 15 2021, David Hildenbrand <david@redhat.com> wrote: >> >>> On 15.07.21 11:30, Cornelia Huck wrote: >>>> On Thu, Jul 15 2021, David Hildenbrand <david@redhat.com> wrote: >>>> >>>>> On 14.07.21 17:25, Pierre Morel wrote: >>>>>> STSI(15.1.x) gives information on the CPU configuration topology. >>>>>> Let's accept the interception of STSI with the function code 15 and >>>>>> let the userland part of the hypervisor handle it. >>>>>> >>>>>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com> >>>>>> --- >>>>>> arch/s390/kvm/priv.c | 11 ++++++++++- >>>>>> 1 file changed, 10 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c >>>>>> index 9928f785c677..4ab5f8b7780e 100644 >>>>>> --- a/arch/s390/kvm/priv.c >>>>>> +++ b/arch/s390/kvm/priv.c >>>>>> @@ -856,7 +856,7 @@ static int handle_stsi(struct kvm_vcpu *vcpu) >>>>>> if (vcpu->arch.sie_block->gpsw.mask & PSW_MASK_PSTATE) >>>>>> return kvm_s390_inject_program_int(vcpu, >>>>>> PGM_PRIVILEGED_OP); >>>>>> - if (fc > 3) { >>>>>> + if (fc > 3 && fc != 15) { >>>>>> kvm_s390_set_psw_cc(vcpu, 3); >>>>>> return 0; >>>>>> } >>>>>> @@ -893,6 +893,15 @@ static int handle_stsi(struct kvm_vcpu *vcpu) >>>>>> goto out_no_data; >>>>>> handle_stsi_3_2_2(vcpu, (void *) mem); >>>>>> break; >>>>>> + case 15: >>>>>> + if (sel1 != 1 || sel2 < 2 || sel2 > 6) >>>>>> + goto out_no_data; >>>>>> + if (vcpu->kvm->arch.user_stsi) { >>>>>> + insert_stsi_usr_data(vcpu, operand2, ar, fc, sel1, >>>>>> sel2); >>>>>> + return -EREMOTE; >>>> >>>> This bypasses the trace event further down. >>>> >>>>>> + } >>>>>> + kvm_s390_set_psw_cc(vcpu, 3); >>>>>> + return 0; >>>>>> } >>>>>> if (kvm_s390_pv_cpu_is_protected(vcpu)) { >>>>>> memcpy((void *)sida_origin(vcpu->arch.sie_block), >>>>>> (void *)mem, >>>>> 3. User space awareness >>>>> >>>>> How can user space identify that we actually forward these intercepts? >>>>> How can it enable them? The old KVM_CAP_S390_USER_STSI capability >>>>> is not sufficient. >>>> >>>> Why do you think that it is not sufficient? USER_STSI basically says >>>> "you may get an exit that tells you about a buffer to fill in some more >>>> data for a stsi command, and we also tell you which call". If userspace >>>> does not know what to add for a certain call, it is free to just do >>>> nothing, and if it does not get some calls it would support, that >>>> should >>>> not be a problem, either? >>> >>> If you migrate your VM from machine a to machine b, from kernel a to >>> kernel b, and kernel b does not trigger exits to user space for fc=15, >>> how could QEMU spot and catch the different capabilities to make sure >>> the guest can continue using the feature? >> >> Wouldn't that imply that the USER_STSI feature, in the function-agnostic >> way it is documented, was broken from the start? > > Likely. We should have forwarded everything to user space most probably > and not try being smart in the kernel. > >> >> Hm. Maybe we need some kind of facility where userspace can query the >> kernel and gets a list of the stsi subcodes it may get exits for, and >> possibly fail to start the migration. Having a new capability to be >> enabled for every new subcode feels like overkill. I don't think we can >> pass a payload ("enable these subfunctions") to a cap. > > Maybe a new capability that forwards everything to user space when > enabled, and lets user space handle errors. > > Or a specialized one only to unlock fc=15. >
I think the lack of a good comment in patch 2/2 is the problem. Facility 11 belong to CPU model and enables both the STSI 15 and the PFT instructions
Sorry about that.
-- Pierre Morel IBM Lab Boeblingen
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