linux-kernel - Re: [PATCH v5 11/21] KVM: s390: pci: do initial setup for AEN interpretation

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Date:   Wed, 20 Apr 2022 09:43:40 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>, linux-s390@...r.kernel.org
Cc:     alex.williamson@...hat.com, cohuck@...hat.com,
        schnelle@...ux.ibm.com, farman@...ux.ibm.com,
        borntraeger@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, frankja@...ux.ibm.com, david@...hat.com,
        imbrenda@...ux.ibm.com, vneethv@...ux.ibm.com,
        oberpar@...ux.ibm.com, freude@...ux.ibm.com, thuth@...hat.com,
        pasic@...ux.ibm.com, pbonzini@...hat.com, corbet@....net,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 11/21] KVM: s390: pci: do initial setup for AEN
 interpretation

On 4/19/22 4:16 AM, Pierre Morel wrote:
> 
> 
> On 4/4/22 19:43, Matthew Rosato wrote:
>> Initial setup for Adapter Event Notification Interpretation for zPCI
>> passthrough devices.  Specifically, allocate a structure for 
>> forwarding of
>> adapter events and pass the address of this structure to firmware.
>>
>> Signed-off-by: Matthew Rosato <mjrosato@...ux.ibm.com>
>> ---

...

>> +
>> +static int zpci_reset_aipb(u8 nisc)
>> +{
>> +    /*
>> +     * AEN registration can only happen once per system boot.  If
>> +     * an aipb already exists then AEN was already registered and
>> +     * we can re-use the aipb contents.  This can only happen if
>> +     * the KVM module was removed and re-inserted.
>> +     */
>> +    if (zpci_aipb->aipb.faal != ZPCI_NR_DEVICES ||
>> +        zpci_aipb->aipb.afi != nisc) {
>> +        return -EINVAL;
>> +    }
> 
> I do not understand how faal cound be different of ZPCI_NR_DEVICES if 
> aipb has been already initialised.
> Same for afi.
> Can you please explain?

Well, my concern was along the lines of 'what if we rmmod kvm and then 
insmod a different version of the kvm module' -- These are really sanity 
checks.

Now, ZPCI_NR_DEVICES/faal is built in with PCI, so yeah this check is 
probably unnecessary as we shouldn't be able to change this value 
without a new kernel.

afi is however derived from nisc, which was passed in all the way from 
kvm_s390_gib_init during kvm_arch_init.  Today, this is hard-coded as 
GAL_ISC; but the point is that this is hard-coded within the kvm module, 
so we can't be quite sure that it's the same value every time we insmod 
kvm.  In an (admittedly, far-fetched) scenario where we insmod kvm, 
initialize AEN with GAL_ISC, rmmod kvm, then insmod a kvm where, for 
example, GAL_ISC was changed to a different number, we would need to 
trigger a failure here because we have no way to update the forwarding 
isc with firmware until the kernel is rebooted.