Re: [RFC PATCH v3 2/2] KVM: s390: Extend the USER_SIGP capability

Janosch Frank
2021-11-11T16:13:12Z

On 11/11/21 16:03, Eric Farman wrote:
> On Thu, 2021-11-11 at 10:15 +0100, David Hildenbrand wrote:
>> On 10.11.21 21:33, Eric Farman wrote:
>>> With commit 2444b352c3ac ("KVM: s390: forward most SIGP orders to
>>> user
>>> space") we have a capability that allows the "fast" SIGP orders (as
>>> defined by the Programming Notes for the SIGNAL PROCESSOR
>>> instruction in
>>> the Principles of Operation) to be handled in-kernel, while all
>>> others are
>>> sent to userspace for processing.
>>>
>>> This works fine but it creates a situation when, for example, a
>>> SIGP SENSE
>>> might return CC1 (STATUS STORED, and status bits indicating the
>>> vcpu is
>>> stopped), when in actuality userspace is still processing a SIGP
>>> STOP AND
>>> STORE STATUS order, and the vcpu is not yet actually stopped. Thus,
>>> the
>>> SIGP SENSE should actually be returning CC2 (busy) instead of CC1.
>>>
>>> To fix this, add another CPU capability, dependent on the USER_SIGP
>>> one,
>>> and two associated IOCTLs. One IOCTL will be used by userspace to
>>> mark a
>>> vcpu "busy" processing a SIGP order, and cause concurrent orders
>>> handled
>>> in-kernel to be returned with CC2 (busy). Another IOCTL will be
>>> used by
>>> userspace to mark the SIGP "finished", and the vcpu free to process
>>> additional orders.
>>>
>>
>> This looks much cleaner to me, thanks!
>>
>> [...]
>>
>>> diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
>>> index c07a050d757d..54371cede485 100644
>>> --- a/arch/s390/kvm/kvm-s390.h
>>> +++ b/arch/s390/kvm/kvm-s390.h
>>> @@ -82,6 +82,22 @@ static inline int is_vcpu_idle(struct kvm_vcpu
>>> *vcpu)
>>> return test_bit(vcpu->vcpu_idx, vcpu->kvm->arch.idle_mask);
>>> }
>>>
>>> +static inline bool kvm_s390_vcpu_is_sigp_busy(struct kvm_vcpu
>>> *vcpu)
>>> +{
>>> + return (atomic_read(&vcpu->arch.sigp_busy) == 1);
>>
>> You can drop ()
>>
>>> +}
>>> +
>>> +static inline bool kvm_s390_vcpu_set_sigp_busy(struct kvm_vcpu
>>> *vcpu)
>>> +{
>>> + /* Return zero for success, or -EBUSY if another vcpu won */
>>> + return (atomic_cmpxchg(&vcpu->arch.sigp_busy, 0, 1) == 0) ? 0 :
>>> -EBUSY;
>>
>> You can drop () as well.
>>
>> We might not need the -EBUSY semantics after all. User space can just
>> track if it was set, because it's in charge of setting it.
>
> Hrm, I added this to distinguish a newer kernel with an older QEMU, but
> of course an older QEMU won't know the difference either. I'll
> doublecheck that this is works fine in the different permutations.
>
>>
>>> +}
>>> +
>>> +static inline void kvm_s390_vcpu_clear_sigp_busy(struct kvm_vcpu
>>> *vcpu)
>>> +{
>>> + atomic_set(&vcpu->arch.sigp_busy, 0);
>>> +}
>>> +
>>> static inline int kvm_is_ucontrol(struct kvm *kvm)
>>> {
>>> #ifdef CONFIG_KVM_S390_UCONTROL
>>> diff --git a/arch/s390/kvm/sigp.c b/arch/s390/kvm/sigp.c
>>> index 5ad3fb4619f1..a37496ea6dfa 100644
>>> --- a/arch/s390/kvm/sigp.c
>>> +++ b/arch/s390/kvm/sigp.c
>>> @@ -276,6 +276,10 @@ static int handle_sigp_dst(struct kvm_vcpu
>>> *vcpu, u8 order_code,
>>> if (!dst_vcpu)
>>> return SIGP_CC_NOT_OPERATIONAL;
>>>
>>> + if (kvm_s390_vcpu_is_sigp_busy(dst_vcpu)) {
>>> + return SIGP_CC_BUSY;
>>> + }
>>
>> You can drop {}
>
> Arg, I had some debug in there which needed the braces, and of course
> it's unnecessary now. Thanks.
>
>>
>>> +
>>> switch (order_code) {
>>> case SIGP_SENSE:
>>> vcpu->stat.instruction_sigp_sense++;
>>> @@ -411,6 +415,12 @@ int kvm_s390_handle_sigp(struct kvm_vcpu
>>> *vcpu)
>>> if (handle_sigp_order_in_user_space(vcpu, order_code,
>>> cpu_addr))
>>> return -EOPNOTSUPP;
>>>
>>> + /* Check the current vcpu, if it was a target from another vcpu
>>> */
>>> + if (kvm_s390_vcpu_is_sigp_busy(vcpu)) {
>>> + kvm_s390_set_psw_cc(vcpu, SIGP_CC_BUSY);
>>> + return 0;
>>> + }
>>
>> I don't think we need this. I think the above (checking the target of
>> a
>> SIGP order) is sufficient. Or which situation do you have in mind?
>>
>
> Hrm... I think you're right. I was thinking of this:
>
> VCPU 1 - SIGP STOP CPU 2
> VCPU 2 - SIGP SENSE CPU 1
>
> But of course either CPU2 is going to be marked "busy" first, and the
> sense doesn't get processed until it's reset, or the sense arrives
> first, and the busy/notbusy doesn't matter. Let me doublecheck my tests
> for the non-RFC version.
>
>>
>>
>> I do wonder if we want to make this a kvm_arch_vcpu_ioctl() instead,
>
> In one of my original attempts between v1 and v2, I had put this there.
> This reliably deadlocks my guest, because the caller (kvm_vcpu_ioctl())
> tries to acquire vcpu->mutex, and racing SIGPs (via KVM_RUN) might
> already be holding it. Thus, it's an async ioctl. I could fold it into
> the existing interrupt ioctl, but as those are architected structs it
> seems more natural do it this way. Or I have mis-understood something
> along the way?
>
>> essentially just providing a KVM_S390_SET_SIGP_BUSY *and* providing
>> the
>> order. "order == 0" sets it to !busy.
>
> I'd tried this too, since it provided some nice debug-ability.
> Unfortunately, I have a testcase (which I'll eventually get folded into
> kvm-unit-tests :)) that picks a random order between 0-255, knowing
> that there's only a couple handfuls of valid orders, to check the
> response. Zero is valid architecturally (POPS figure 4-29), even if
> it's unassigned. The likelihood of it becoming assigned is probably
> quite low, but I'm not sure that I like special-casing an order of zero
> in this way.
>

Looking at the API I'd like to avoid having two IOCTLs and I'd love to
see some way to extend this without the need for a whole new IOCTL.

>> Not that we would need the value
>> right now, but who knows for what we might reuse that interface in
>> the
>> future.
>>
>> Thanks!
>>
>