linux-kernel - Re: [PATCH v5 06/13] KVM: s390: interfaces to manage guest's AP matrix

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Date:   Wed, 16 May 2018 10:29:10 -0400
From:   Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pmorel@...ux.vnet.ibm.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com
Subject: Re: [PATCH v5 06/13] KVM: s390: interfaces to manage guest's AP
 matrix

On 05/11/2018 12:08 PM, Halil Pasic wrote:
>
>
> On 05/07/2018 05:11 PM, Tony Krowiak wrote:
>> Provides interfaces to manage the AP adapters, usage domains
>> and control domains assigned to a KVM guest.
>>
>> The guest's SIE state description has a satellite structure called the
>> Crypto Control Block (CRYCB) containing three bitmask fields
>> identifying the adapters, queues (domains) and control domains
>> assigned to the KVM guest:
>
> [..]
>
>> index 00bcfb0..98b53c7 100644
>> --- a/arch/s390/kvm/kvm-ap.c
>> +++ b/arch/s390/kvm/kvm-ap.c
>> @@ -7,6 +7,7 @@
>
> [..]
>
>> +
>> +/**
>> + * kvm_ap_validate_queue_sharing
>> + *
>> + * Verifies that the APQNs derived from the cross product of the AP 
>> adapter IDs
>> + * and AP queue indexes comprising the AP matrix are not configured for
>> + * another guest. AP queue sharing is not allowed.
>> + *
>> + * @kvm: the KVM guest
>> + * @matrix: the AP matrix
>> + *
>> + * Returns 0 if the APQNs are valid, otherwise; returns -EBUSY.
>> + */
>> +static int kvm_ap_validate_queue_sharing(struct kvm *kvm,
>> +                     struct kvm_ap_matrix *matrix)
>> +{
>> +    struct kvm *vm;
>> +    unsigned long *apm, *aqm;
>> +    unsigned long apid, apqi;
>> +
>> +
>> +    /* No other VM may share an AP Queue with the input VM */
>> +    list_for_each_entry(vm, &vm_list, vm_list) {
>> +        if (kvm == vm)
>> +            continue;
>> +
>> +        apm = kvm_ap_get_crycb_apm(vm);
>> +        if (!bitmap_and(apm, apm, matrix->apm, matrix->apm_max + 1))
>> +            continue;
>> +
>> +        aqm = kvm_ap_get_crycb_aqm(vm);
>> +        if (!bitmap_and(aqm, aqm, matrix->aqm, matrix->aqm_max + 1))
>> +            continue;
>> +
>> +        for_each_set_bit_inv(apid, apm, matrix->apm_max + 1)
>> +            for_each_set_bit_inv(apqi, aqm, matrix->aqm_max + 1)
>> +                kvm_ap_log_sharing_err(vm, apid, apqi);
>> +
>> +        return -EBUSY;
>> +    }
>> +
>> +    return 0;
>> +}
>> +
>> +int kvm_ap_configure_matrix(struct kvm *kvm, struct kvm_ap_matrix 
>> *matrix)
>> +{
>> +    int ret = 0;
>> +
>> +    mutex_lock(&kvm->lock);
>
> You seem to take only kvm->lock, vm_list however (used in
> kvm_ap_validate_queue_sharing()) seems to be protected by
> kvm_lock.
>
> Can you tell me why is this supposed to be safe?
>
> What is supposed to prevent an execution like
> vm1: call kvm_ap_configure_matrix(m1)
> vm2: call kvm_ap_configure_matrix(m2)
> vm1: call kvm_ap_validate_queue_sharing(m1)
> vm2: call kvm_ap_validate_queue_sharing(m2)
> vm1: call kvm_ap_set_crycb_masks(m1)
> vm2: call kvm_ap_set_crycb_masks(m2)
>
> where, let's say, m1 and m2 are equal in the sense that the
> mask values are the same?

vm1 will get the kvm->lock first in your scenario when
kvm_ap_configure_matrix(m1) is invoked. Since the other
functions - i.e., kvm_ap_validate_queue_sharing(m1) and
kvm_ap_set_crycb_masks(m1) - are static and only called
from the kvm_ap_configure_matrix(m1), your scenario
can never happen because vm2 will not get the lock until
kvm_ap_configure_matrix(m1) has completed. I see your
point, however, and maybe I should also acquire the kvm_lock.

>
>
> Regards,
> Halil
>
>> +
>> +    ret = kvm_ap_validate_queue_sharing(kvm, matrix);
>> +    if (ret)
>> +        goto done;
>> +
>> +    kvm_ap_set_crycb_masks(kvm, matrix);
>> +
>> +done:
>> +    mutex_unlock(&kvm->lock);
>> +
>> +    return ret;
>> +}
>> +EXPORT_SYMBOL(kvm_ap_configure_matrix);
>> +