linux-kernel - Re: [PATCH v10 2/3] KVM: s390: guest support for topology function

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Date:   Tue, 28 Jun 2022 16:13:00 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.ibm.com, cohuck@...hat.com,
        david@...hat.com, thuth@...hat.com, imbrenda@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com, wintera@...ux.ibm.com,
        seiden@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v10 2/3] KVM: s390: guest support for topology function



On 6/28/22 14:18, Janis Schoetterl-Glausch wrote:
> On 6/28/22 12:58, Pierre Morel wrote:
>>
>>
>> On 6/28/22 10:59, Janis Schoetterl-Glausch wrote:
>>> On 6/20/22 14:54, Pierre Morel wrote:
>>>> We report a topology change to the guest for any CPU hotplug.
>>>>
>>>> The reporting to the guest is done using the Multiprocessor
>>>> Topology-Change-Report (MTCR) bit of the utility entry in the guest's
>>>> SCA which will be cleared during the interpretation of PTF.
>>>>
>>>> On every vCPU creation we set the MCTR bit to let the guest know the
>>>> next time he uses the PTF with command 2 instruction that the
>>>> topology changed and that he should use the STSI(15.1.x) instruction
>>>> to get the topology details.
>>>>
>>>> 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 when userland
>>>> support the CPU Topology facility.
>>>>
>>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>>> ---
>>>>    arch/s390/include/asm/kvm_host.h | 11 ++++++++---
>>>>    arch/s390/kvm/kvm-s390.c         | 27 ++++++++++++++++++++++++++-
>>>>    arch/s390/kvm/priv.c             | 15 +++++++++++----
>>>>    arch/s390/kvm/vsie.c             |  3 +++
>>>>    4 files changed, 48 insertions(+), 8 deletions(-)
>>>>
>>> [...]
>>>
>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>> index 8fcb56141689..95b96019ca8e 100644
>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>> @@ -1691,6 +1691,25 @@ static int kvm_s390_get_cpu_model(struct kvm *kvm, struct kvm_device_attr *attr)
>>>>        return ret;
>>>>    }
>>>>
>>>> +/**
>>>> + * kvm_s390_sca_set_mtcr
>>>
>>> I wonder if there is a better name, kvm_s390_report_topology_change maybe?
>>>
>>>> + * @kvm: guest KVM description
>>>> + *
>>>> + * Is only relevant if the topology facility is present,
>>>> + * the caller should check KVM facility 11
>>>> + *
>>>> + * Updates the Multiprocessor Topology-Change-Report to signal
>>>> + * the guest with a topology change.
>>>> + */
>>>> +static void kvm_s390_sca_set_mtcr(struct kvm *kvm)
>>>> +{
>>>
>>> Do we need a sca_lock read_section here? If we don't why not?
>>> Did not see one up the stack, but I might have overlooked something.
>>
>> Yes we do.
>> As I said about your well justified comment in a previous mail, ipte_lock is not the right thing to use here and I will replace with an inter locked update.
> 
> Not sure I'm understanding you right, you're saying we need both? i.e.:
> 
> struct bsca_block *sca;
> 
> read_lock(&vcpu->kvm->arch.sca_lock);
> sca = kvm->arch.sca;
> atomic_or(SCA_UTILITY_MTCR, &sca->utility);
> read_unlock(&vcpu->kvm->arch.sca_lock);
> 
> Obviously you would need to change the definition of the utility field and could not use a bit field like Janosch
> suggested, unless you want to use a cmpxchg loop.
> It's a bit ugly that utility is a two byte value.
> Maybe there is a nicer way to set that bit, OR (OI, OIY) seem appropriate, but I don't know if they have a nice
> abstraction in Linux or if you'd need inline asm.

I was think to something like this because it is what is used most of 
the time when a bit is to be change concurrently with firmware.


+union sca_utility {
+       __u16 val;
+       struct {
+               __u16 mtcr : 1;
+               __u16 reserved : 15;
+       };
+};
+
  struct bsca_block {
         union ipte_control ipte_control;
         __u64   reserved[5];
         __u64   mcn;
-       __u64   reserved2;
+       union sca_utility utility;
+       __u8    reserved2[6];
         struct bsca_entry cpu[KVM_S390_BSCA_CPU_SLOTS];
  }

....

static void kvm_s390_sca_set_mtcr(struct kvm *kvm, int val)
{
         struct bsca_block *sca = kvm->arch.sca;
         union sca_utility new, old;

         read_lock(&kvm->arch.sca_lock);
         do {
                 old = READ_ONCE(sca->utility);
                 new = old;
                 new.mtcr = val ? 1 : 0;
         } while (cmpxchg(&sca->utility.val, old.val, new.val) != old.val);
         read_lock(&kvm->arch.sca_lock);
}

>>
>>>
>>>> +    struct bsca_block *sca = kvm->arch.sca; /* SCA version doesn't matter */
>>>> +
>>>> +    ipte_lock(kvm);
>>>> +    sca->utility |= SCA_UTILITY_MTCR;
>>>> +    ipte_unlock(kvm);
>>>> +}
>>>> +
>>>
>>> [...]
>>>
>>
> 

-- 
Pierre Morel
IBM Lab Boeblingen