[Qemu-arm] Can we boot a 512U kvm guest?

[Zenghui Yu]

Hi folks,

Since commit e25028c8ded0 ("KVM: arm/arm64: Bump VGIC_V3_MAX_CPUS to
512"), we seemed to be allowed to boot a 512U guest.  But I failed to
start it up with the latest QEMU.  I guess there are at least *two*
reasons (limitations).

First I got a QEMU abort:
        "kvm_set_irq: Invalid argument"

Enable the trace_kvm_irq_line() under debugfs, when it comed with
vcpu-256, I got:
        "Inject UNKNOWN interrupt (3), vcpu->idx: 0, num: 23, level: 0"
and kvm_vm_ioctl_irq_line() returns -EINVAL to user-space...

So the thing is that we only have 8 bits for vcpu_index field ([23:16])
in KVM_IRQ_LINE ioctl.  irq_type field will be corrupted if we inject a
PPI to vcpu-256, whose vcpu_index will take 9 bits.

I temporarily patched the KVM and QEMU with the following diff:

---8<---
diff --git a/arch/arm64/include/uapi/asm/kvm.h 
b/arch/arm64/include/uapi/asm/kvm.h
index 95516a4..39a0fb1 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -325,10 +325,10 @@ struct kvm_vcpu_events {
 #define   KVM_ARM_VCPU_TIMER_IRQ_PTIMER                1

 /* KVM_IRQ_LINE irq field index values */
-#define KVM_ARM_IRQ_TYPE_SHIFT         24
-#define KVM_ARM_IRQ_TYPE_MASK          0xff
+#define KVM_ARM_IRQ_TYPE_SHIFT         28
+#define KVM_ARM_IRQ_TYPE_MASK          0xf
 #define KVM_ARM_IRQ_VCPU_SHIFT         16
-#define KVM_ARM_IRQ_VCPU_MASK          0xff
+#define KVM_ARM_IRQ_VCPU_MASK          0xfff
 #define KVM_ARM_IRQ_NUM_SHIFT          0
 #define KVM_ARM_IRQ_NUM_MASK           0xffff

---8<---

It makes things a bit better, it also immediately BREAKs the api with
old versions.


Next comes one more QEMU abort (with the "fix" above):
        "Failed to set device address: No space left on device"

We register two io devices (rd_dev and sgi_dev) on KVM_MMIO_BUS for
each redistributor. 512 vcpus take 1024 io devices, which is beyond the
maximum limitation of the current kernel - NR_IOBUS_DEVS (1000).
So we get a ENOSPC error here.


I don't know if the similar problems have been discussed before in ML.
Is it time to really support the 512U guest?


Thanks,
zenghui