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
