Re: [Qemu-arm] [PATCH] KVM: arm/arm64: vgic: Allow more than 256 vcpus for KVM_IRQ_LINE

[Will Deacon]

On Sun, Aug 18, 2019 at 03:07:10PM +0100, Marc Zyngier wrote:
> While parts of the VGIC support a large number of vcpus (we
> bravely allow up to 512), other parts are more limited.
> 
> One of these limits is visible in the KVM_IRQ_LINE ioctl, which
> only allows 256 vcpus to be signalled when using the CPU or PPI
> types. Unfortunately, we've cornered ourselves badly by allocating
> all the bits in the irq field.
> 
> Since the irq_type subfield (8 bit wide) is currently only taking
> the values 0, 1 and 2 (and we have been careful not to allow anything
> else), let's reduce this field to only 4 bits, and allocate the
> remaining 4 bits to a vcpu2_index, which acts as a multiplier:
> 
>   vcpu_id = 256 * vcpu2_index + vcpu_index
> 
> With that, and a new capability (KVM_CAP_ARM_IRQ_LINE_LAYOUT_2)
> allowing this to be discovered, it becomes possible to inject
> PPIs to up to 4096 vcpus. But please just don't.

Do you actually need a new capability for this? Older kernels reject
non-zero upper bits in the 'irq_type', so isn't that enough to probe
for this directly?

Will