Re: Can we boot a 512U kvm guest?

[Auger Eric <eric.auger@redhat.com>]

Hi,

On 8/13/19 4:17 PM, Marc Zyngier wrote:
> 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...).

My bad. At that time I was able to test with 224 vcpus with QEMU (256
vcpus were tested as well) and I failed to see those other limitations,
thinking the only limitation left was the number of redistributor
regions we could register.
> 
>>
>> 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.

The initial goal was to keep up with the HPC physical cpu count increase
and allow similar numbers as x86 supports.

Thanks

Eric (with limited internet access atm)

> 
> Thanks,
> 
> 	M.
> 
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