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

Janosch Frank
2021-11-12T08:49:29Z

On 11/11/21 18:48, Eric Farman wrote:
> On Thu, 2021-11-11 at 17:13 +0100, Janosch Frank wrote:
>> 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
>
> Since the order is a single byte, we could have the payload of an ioctl
> say "0-255 is an order that we're busy processing, anything higher than
> that resets the busy" or something. That would remove the need for a
> second IOCTL.
>
>> and I'd love to
>> see some way to extend this without the need for a whole new IOCTL.
>>
>
> Do you mean zero IOCTLs? Because... I think the only way we can do that
> is to get rid of USER_SIGP altogether, and handle everything in-kernel.
> Some weeks ago I played with QEMU not enabling USER_SIGP, but I can't
> say I've tried it since we went down this "mark a vcpu busy" path. If I
> do that busy/not-busy tagging in the kernel for !USER_SIGP, that might
> not be a bad thing anyway. But I don't know how we get the behavior
> straightened out for USER_SIGP without some type of handshake.

I'd move over to a very small struct argument with a command and a flags
field so we can extend the IOCTL at a later time without the need to
introduce a new IOCTL.

IMHO there's no real need to make this IOCTL as small as possible and
only handle an int as the argument with < 0 shenanigans. We should
rather focus on making this a nice and sane API if we have the option to
do so.

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