Re: [RFC PATCH] KVM: arm/arm64: GICv4: Support shared VLPI

[Kunkun Jiang]

Hi Marc,

On 2023/11/6 23:33, Marc Zyngier wrote:
> On Mon, 06 Nov 2023 14:59:01 +0000,
> Kunkun Jiang <jiangkunkun@xxxxxxxxxx> wrote:
> > The virtio-pci driver write entry1-6
> > massage.data in the msix-table and trap to QEMU for processing. The
> > massage.data is as follow:
> > > entry-0 0
> > > entry-1 1
> > > entry-2 1
> > > entry-3 1
> > > entry-4 1
> > > entry-5 1
> > > entry-6 1
> Urgh... is vp_modern_queue_vector() used in your configuration? This
> is ... terrible.
I encountered this problem using the 4.19 version kernel, but not the
5.10 version. This vp_modern_queue_vector() function does not exist
in 4.19, but it uses 'vp_iowrite16(msix_vec, &cfg->queue_msix_vector)',
the same as vp_modern_queue_vector().

In the past two days, I learned about the virtio driver and made some
new discoveries. When 'num_queues' is greater than maxcpus, it will
fall back into MSI-X with one shared for queues. The two patches[1],
submitted by Dongli, limits the number of hw queues used by
virtio-blk/virtio-scsi by 'nr_cpu_ids'. The two patches were merged
in 5.1-rc2. And the patch related virtio-blk was merged into the 4.19
stable branch.The patch related virtio-scsi was not merged.
[1] 
https://lore.kernel.org/all/1553682995-5682-1-git-send-email-dongli.zhang@xxxxxxxxxx/

This is the earliest discussion.
https://lore.kernel.org/all/e4afe4c5-0262-4500-aeec-60f30734b4fc@default/

I don't know if there are other circumstances that would cause it to
fall back into MSI-X with one shared for queues. At least the hack
method is possible.
> I wonder if PCIe actually allows this sort of thing.
Do you think the virtio driver should be modified?
> In any case, this sort of behaviour breaks so many thing in KVM's
> implementation that I'd recommend you disable GICv4 until we have a
> good solution for that.
There seems to be no restriction in the GIC specification that multiple
host irqs cannot be mapped to the same vlpi. Or maybe I didn't notice.
Do you think there are any risks?

GICv3 does not have this issue, but is this configuration legal?

Thanks,
Kunkun Jiang