linux-kernel - Re: [QUESTION] Exception print when enabling GICv4

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Message-ID: <87bktu1hfj.wl-maz@kernel.org>
Date:   Tue, 12 Jul 2022 16:25:52 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     "chenxiang (M)" <chenxiang66@...ilicon.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
        chenxiang via <qemu-devel@...gnu.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [QUESTION] Exception print when enabling GICv4

Hi Xiang,

On Tue, 12 Jul 2022 13:55:16 +0100,
"chenxiang (M)" <chenxiang66@...ilicon.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.

Thanks,

	M.

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