Re: [QUESTION] Exception print when enabling GICv4

[Marc Zyngier]

On Wed, 13 Jul 2022 07:02:10 +0100,
"chenxiang (M)" <chenxiang66@hisilicon.com> wrote:
> 
> Hi Marc,
> 
> Thank you for your reply.
> 
> 在 2022/7/12 23:25, Marc Zyngier 写道:
> > Hi Xiang,
> > 
> > On Tue, 12 Jul 2022 13:55:16 +0100,
> > "chenxiang (M)" <chenxiang66@hisilicon.com> wrote:
> >> Hi,
> >> I encounter a issue related to GICv4 enable on ARM64 platform (kernel
> >> 5.19-rc4, qemu 6.2.0):
> >> We have a accelaration module whose VF has 3 MSI interrupts, and we
> >> passthrough it to virtual machine with following steps:
> >> 
> >> echo 0000:79:00.1 > /sys/bus/pci/drivers/hisi_hpre/unbind
> >> echo vfio-pci >
> >> /sys/devices/pci0000\:78/0000\:78\:00.0/0000\:79\:00.1/driver_override
> >> echo 0000:79:00.1 > /sys/bus/pci/drivers_probe
> >> 
> >> Then we boot VM with "-device vfio-pci,host=79:00.1,id=net0 \".
> >> When insmod the driver which registers 3 PCI MSI interrupts in VM,
> >> some exception print occur as following:
> >> 
> >> vfio-pci 0000:3a:00.1: irq bypass producer (token 000000008f08224d)
> >> registration fails: 66311
> >> 
> >> I find that bit[6:4] of register PCI_MSI_FLAGS is 2 (4 MSI interrupts)
> >> though we only register 3 PCI MSI interrupt,
> >> 
> >> and only 3 MSI interrupt is activated at last.
> >> It allocates 4 vectors in function vfio_msi_enable() (qemu)  as it
> >> reads the register PCI_MSI_FLAGS.
> >> Later it will  call system call VFIO_DEVICE_SET_IRQS to set forwarding
> >> for those interrupts
> >> using function kvm_vgic_v4_set_forrwarding() as GICv4 is enabled. For
> >> interrupt 0~2, it success to set forwarding as they are already
> >> activated,
> >> but for the 4th interrupt, it is not activated, so ite is not found in
> >> function vgic_its_resolve_lpi(), so above printk occurs.
> >> 
> >> It seems that we only allocate and activate 3 MSI interrupts in guest
> >> while it tried to set forwarding for 4 MSI interrupts in host.
> >> Do you have any idea about this issue?
> > I have a hunch: QEMU cannot know that the guest is only using 3 MSIs
> > out of the 4 that the device can use, and PCI/Multi-MSI only has a
> > single enable bit for all MSIs. So it probably iterates over all
> > possible MSIs and enable the forwarding. Since the guest has only
> > created 3 mappings in the virtual ITS, the last call fails. I would
> > expect the guest to still work properly though.
> 
> Yes, that's the reason of exception print.
> Is it possible for QEMU to get the exact number of interrupts guest is
> using? It seems not.

Not really. Or rather, this is a pretty involved process: you'd need
to stop the guest, perform a save operation on the ITS (as if you were
doing a migration), and then introspect the ITS tables to find whether
there is a mapping for each of the possible events generated by the
device. Clearly, that's overkill.

A better approach would be to be able to retrieve an individual
mapping, using a new API that would be similar to KVM_SIGNAL_MSI. It
would take the same kvm_msi structure as input, and retrieving the
{LPI, CPU} pair or an error if there is no mapping.

Thanks,

        M.

-- 
Without deviation from the norm, progress is not possible.