Messages in this thread | | | Subject | Re: [PATCH v1 1/2] s390x: KVM: accept STSI for CPU topology information | From | David Hildenbrand <> | Date | Thu, 15 Jul 2021 12:19:05 +0200 |
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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.
-- Thanks,
David / dhildenb
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