Re: Can we boot a 512U kvm guest?

[Marc Zyngier <marc.zyngier@arm.com>]

On Tue, 13 Aug 2019 09:50:27 +0100,
Zenghui Yu <yuzenghui@huawei.com> wrote:

Hi Zenghui,

> 
> 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.

Irk. Not great indeed. Clearly, we have a couple of holes in the way
we test these ABI changes (/me eyes Eric...).

> 
> 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.

Yes, and we can't have that (specially if you consider that this API
is shared between 32 and 64bit). One "get out of jail card" is to
steal a few bits from the top of the word, and encode things there:

diff --git a/arch/arm/include/uapi/asm/kvm.h b/arch/arm/include/uapi/asm/kvm.h
index 4602464ebdfb..86db092e4c2f 100644
--- a/arch/arm/include/uapi/asm/kvm.h
+++ b/arch/arm/include/uapi/asm/kvm.h
@@ -254,8 +254,10 @@ struct kvm_vcpu_events {
 #define   KVM_DEV_ARM_ITS_CTRL_RESET		4
 
 /* KVM_IRQ_LINE irq field index values */
+#define KVM_ARM_IRQ_VCPU2_SHIFT		28
+#define KVM_ARM_IRQ_VCPU2_MASK		0xf
 #define KVM_ARM_IRQ_TYPE_SHIFT		24
-#define KVM_ARM_IRQ_TYPE_MASK		0xff
+#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_NUM_SHIFT		0
diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index 7b7ac0f6cec9..44cb25bfc95e 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -308,8 +308,10 @@ struct kvm_vcpu_events {
 #define   KVM_ARM_VCPU_TIMER_IRQ_PTIMER		1
 
 /* KVM_IRQ_LINE irq field index values */
+#define KVM_ARM_IRQ_VCPU2_SHIFT		28
+#define KVM_ARM_IRQ_VCPU2_MASK		0xf
 #define KVM_ARM_IRQ_TYPE_SHIFT		24
-#define KVM_ARM_IRQ_TYPE_MASK		0xff
+#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_NUM_SHIFT		0
diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
index 90cedebaeb94..fb685c1c0514 100644
--- a/virt/kvm/arm/arm.c
+++ b/virt/kvm/arm/arm.c
@@ -889,6 +889,7 @@ int kvm_vm_ioctl_irq_line(struct kvm *kvm, struct kvm_irq_level *irq_level,
 
 	irq_type = (irq >> KVM_ARM_IRQ_TYPE_SHIFT) & KVM_ARM_IRQ_TYPE_MASK;
 	vcpu_idx = (irq >> KVM_ARM_IRQ_VCPU_SHIFT) & KVM_ARM_IRQ_VCPU_MASK;
+	vcpu_idx += ((irq >> KVM_ARM_IRQ_VCPU2_SHIFT) & KVM_ARM_IRQ_VCPU2_MASK) * (KVM_ARM_IRQ_VCPU_MASK + 1);
 	irq_num = (irq >> KVM_ARM_IRQ_NUM_SHIFT) & KVM_ARM_IRQ_NUM_MASK;
 
 	trace_kvm_irq_line(irq_type, vcpu_idx, irq_num, irq_level->level);

It should work because we've been careful not to allow value outside
of {0, 1, 2} for irq_type. I don't like it, but I really don't feel
like adding another IRQ related ioctl. We still have 16 irq types
(which is already a waste of space), and we can go up to 4096 vcpu.

Peter, what do you think?

> 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 can reproduce that issue here ("499 vcpus on my Chromebook,
baby"). Not an ABI problem though, and we can bump it up if that's
needed.

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

The real question is "why the hell would you want to do that?" ;-)
Seriously, I'm very interested in finding out what is the use case for
these gigantic VMs, other than debugging the kernel for big machines.

Thanks,

	M.

-- 
Jazz is not dead, it just smells funny.
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