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

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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@xxxxxxxxxxxxx>
---

...

+
+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.



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