Subject: [PATCH v5 08/19] arm/arm64: KVM: make the maximum number of vCPUs a per-VM value

From: [email-masked] (Andre Przywara)
Date: Tue, 06 Jan 2015 14:03:17 +0000

Hi Christoffer,

On 06/01/15 13:29, Christoffer Dall wrote:
> On Mon, Jan 05, 2015 at 05:34:02PM +0000, Andre Przywara wrote:
>> Hi Marc,
>>
>> On 23/12/14 12:07, Marc Zyngier wrote:
>>> On Mon, Dec 08 2014 at 12:37:43 PM, Andre Przywara <[email-masked]> wrote:
>>>> Currently the maximum number of vCPUs supported is a global value
>>>> limited by the used GIC model. GICv3 will lift this limit, but we
>>>> still need to observe it for guests using GICv2.
>>>> So the maximum number of vCPUs is per-VM value, depending on the
>>>> GIC model the guest uses.
>>>> Store and check the value in struct kvm_arch, but keep it down to
>>>> 8 for now.
>>>>
>>>> Signed-off-by: Andre Przywara <[email-masked]>
>>>> ---
>>>> Changelog v4...v5:
>>>> - add define for GIC_V2_MAX_CPUS
>>>> - rename max_hw_vcpus
>>>> - add prototype for ARM non-VGIC configs
>>>>
>>>> Changelog v3...v4:
>>>> - initialize max_vcpus with limit based on host GIC
>>>> - remove *_init_emul_* from VGIC backend
>>>> - refine VCPU limit on VGIC creation
>>>> - print warning when userland tries to create more VCPUs than supported
>>>>
>>>>  arch/arm/include/asm/kvm_host.h   |    1 +
>>>>  arch/arm/kvm/arm.c                |    8 ++++++++
>>>>  arch/arm64/include/asm/kvm_host.h |    3 +++
>>>>  include/kvm/arm_vgic.h            |    8 ++++++++
>>>>  virt/kvm/arm/vgic-v2.c            |    1 +
>>>>  virt/kvm/arm/vgic-v3.c            |    1 +
>>>>  virt/kvm/arm/vgic.c               |   22 ++++++++++++++++++++++
>>>>  7 files changed, 44 insertions(+)
>>>>
>>>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>>>> index b443dfe..7969e6e 100644
>>>> --- a/arch/arm/include/asm/kvm_host.h
>>>> +++ b/arch/arm/include/asm/kvm_host.h
>>>> @@ -68,6 +68,7 @@ struct kvm_arch {
>>>>  
>>>>  	/* Interrupt controller */
>>>>  	struct vgic_dist	vgic;
>>>> +	int max_vcpus;
>>>>  };
>>>>  
>>>>  #define KVM_NR_MEM_OBJS     40
>>>> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
>>>> index 8817fbd..c3d0fbd 100644
>>>> --- a/arch/arm/kvm/arm.c
>>>> +++ b/arch/arm/kvm/arm.c
>>>> @@ -132,6 +132,9 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>>>>  	/* Mark the initial VMID generation invalid */
>>>>  	kvm->arch.vmid_gen = 0;
>>>>  
>>>> +	/* The maximum number of VCPUs is limited by the host's GIC model */
>>>> +	kvm->arch.max_vcpus = kvm_vgic_get_max_vcpus();
>>>> +
>>>>  	return ret;
>>>>  out_free_stage2_pgd:
>>>>  	kvm_free_stage2_pgd(kvm);
>>>> @@ -213,6 +216,11 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, unsigned int id)
>>>>  	int err;
>>>>  	struct kvm_vcpu *vcpu;
>>>>  
>>>> +	if (id >= kvm->arch.max_vcpus) {
>>>> +		err = -EINVAL;
>>>> +		goto out;
>>>> +	}
>>>> +
>>>>  	vcpu = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL);
>>>>  	if (!vcpu) {
>>>>  		err = -ENOMEM;
>>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>>>> index 286bb61..f9e130d 100644
>>>> --- a/arch/arm64/include/asm/kvm_host.h
>>>> +++ b/arch/arm64/include/asm/kvm_host.h
>>>> @@ -59,6 +59,9 @@ struct kvm_arch {
>>>>  	/* VTTBR value associated with above pgd and vmid */
>>>>  	u64    vttbr;
>>>>  
>>>> +	/* The maximum number of vCPUs depends on the used GIC model */
>>>> +	int max_vcpus;
>>>> +
>>>>  	/* Interrupt controller */
>>>>  	struct vgic_dist	vgic;
>>>>  
>>>> diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
>>>> index e691932..72a9fef 100644
>>>> --- a/include/kvm/arm_vgic.h
>>>> +++ b/include/kvm/arm_vgic.h
>>>> @@ -33,6 +33,7 @@
>>>>  #define VGIC_V2_MAX_LRS		(1 << 6)
>>>>  #define VGIC_V3_MAX_LRS		16
>>>>  #define VGIC_MAX_IRQS		1024
>>>> +#define VGIC_V2_MAX_CPUS	8
>>>>  
>>>>  /* Sanity checks... */
>>>>  #if (KVM_MAX_VCPUS > 8)
>>>> @@ -132,6 +133,7 @@ struct vgic_params {
>>>>  	unsigned int	maint_irq;
>>>>  	/* Virtual control interface base address */
>>>>  	void __iomem	*vctrl_base;
>>>> +	int		max_gic_vcpus;
>>>>  };
>>>>  
>>>>  struct vgic_vm_ops {
>>>> @@ -288,6 +290,7 @@ struct kvm_exit_mmio;
>>>>  #ifdef CONFIG_KVM_ARM_VGIC
>>>>  int kvm_vgic_addr(struct kvm *kvm, unsigned long type, u64 *addr, bool write);
>>>>  int kvm_vgic_hyp_init(void);
>>>> +int kvm_vgic_get_max_vcpus(void);
>>>>  int kvm_vgic_init(struct kvm *kvm);
>>>>  int kvm_vgic_create(struct kvm *kvm, u32 type);
>>>>  void kvm_vgic_destroy(struct kvm *kvm);
>>>> @@ -387,6 +390,11 @@ static inline bool vgic_initialized(struct kvm *kvm)
>>>>  {
>>>>  	return true;
>>>>  }
>>>> +
>>>> +static inline int kvm_vgic_get_max_vcpus(void)
>>>> +{
>>>> +	return KVM_MAX_VCPUS;
>>>> +}
>>>>  #endif
>>>>  
>>>>  #endif
>>>> diff --git a/virt/kvm/arm/vgic-v2.c b/virt/kvm/arm/vgic-v2.c
>>>> index e1cd3cb..e8b82b2 100644
>>>> --- a/virt/kvm/arm/vgic-v2.c
>>>> +++ b/virt/kvm/arm/vgic-v2.c
>>>> @@ -237,6 +237,7 @@ int vgic_v2_probe(struct device_node *vgic_node,
>>>>  		 vctrl_res.start, vgic->maint_irq);
>>>>  
>>>>  	vgic->type = VGIC_V2;
>>>> +	vgic->max_gic_vcpus = VGIC_V2_MAX_CPUS;
>>>>  	*ops = &vgic_v2_ops;
>>>>  	*params = vgic;
>>>>  	goto out;
>>>> diff --git a/virt/kvm/arm/vgic-v3.c b/virt/kvm/arm/vgic-v3.c
>>>> index d14c75f..ea39bad 100644
>>>> --- a/virt/kvm/arm/vgic-v3.c
>>>> +++ b/virt/kvm/arm/vgic-v3.c
>>>> @@ -235,6 +235,7 @@ int vgic_v3_probe(struct device_node *vgic_node,
>>>>  	vgic->vcpu_base = vcpu_res.start;
>>>>  	vgic->vctrl_base = NULL;
>>>>  	vgic->type = VGIC_V3;
>>>> +	vgic->max_gic_vcpus = KVM_MAX_VCPUS;
>>>>  
>>>>  	kvm_info("%s@%llx IRQ%d\n", vgic_node->name,
>>>>  		 vcpu_res.start, vgic->maint_irq);
>>>> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
>>>> index c481362..dcdbae5 100644
>>>> --- a/virt/kvm/arm/vgic.c
>>>> +++ b/virt/kvm/arm/vgic.c
>>>> @@ -1848,6 +1848,17 @@ static int vgic_vcpu_init_maps(struct kvm_vcpu *vcpu, int nr_irqs)
>>>>  }
>>>>  
>>>>  /**
>>>> + * kvm_vgic_get_max_vcpus - Get the maximum number of VCPUs allowed by HW
>>>> + *
>>>> + * The host's GIC naturally limits the maximum amount of VCPUs a guest
>>>> + * can use.
>>>> + */
>>>> +int kvm_vgic_get_max_vcpus(void)
>>>> +{
>>>> +	return vgic->max_gic_vcpus;
>>>> +}
>>>> +
>>>> +/**
>>>>   * kvm_vgic_vcpu_init - Initialize per-vcpu VGIC state
>>>>   * @vcpu: pointer to the vcpu struct
>>>>   *
>>>> @@ -2069,6 +2080,8 @@ static int vgic_v2_init_emulation(struct kvm *kvm)
>>>>  	dist->vm_ops.vgic_init = vgic_v2_init;
>>>>  	dist->vm_ops.vgic_init_maps = vgic_v2_init_maps;
>>>>  
>>>> +	kvm->arch.max_vcpus = VGIC_V2_MAX_CPUS;
>>>> +
>>>>  	return 0;
>>>>  }
>>>>  
>>>> @@ -2085,6 +2098,15 @@ static int init_vgic_model(struct kvm *kvm, int type)
>>>>  		break;
>>>>  	}
>>>>  
>>>> +	if (ret)
>>>> +		return ret;
>>>> +
>>>> +	if (atomic_read(&kvm->online_vcpus) > kvm->arch.max_vcpus) {
>>>> +		pr_warn_ratelimited("VGIC model only supports up to %d vCPUs\n",
>>>> +			kvm->arch.max_vcpus);
>>>
>>> I'm not overly keen on the kernel spitting out messages based on
>>> userspace errors. Returning an error should be enough.
>>
>> I added this message after a comment from Christoffer:
>> On 15/10/14 17:27, Christoffer Dall wrote:
>>>>  	case KVM_DEV_TYPE_ARM_VGIC_V2:
>>>> +		nr_vcpus = atomic_read(&kvm->online_vcpus);
>>>> +		if (nr_vcpus > 8)
>>>> +			return false;
>>>> +
>>>
>>> I have a feeling we could be seeing this error a bit, could we
>>> dedicate an error code for the purpose or print a ratelimited warning
>>> or something?
>>
>> Christoffer, can you state whether it's OK to do away with the warning
>> message and just use the -EINVAL?
>>
> 
> we saw these errors from users a lot in the past, hence my comment.
> 
> can you use a different error code?
> 
> otherwise turn it into a kvm_debug().
> 
>>>
>>>> +		ret = -EINVAL;
>>>> +	}
>>>> +
>>>>  	return ret;
>>>>  }
>>>
>>> I'm also slightly confused as to how it works when I create a VM without
>>> an in-kernel GIC. How many vcpus am I allowed?
>>
>> Mmmh, fair point. I guess we should just allow KVM_MAX_VCPUS in this
>> case, right? Has anyone tested userland GIC emulation recently?
>> I will see if I find some method of testing this with QEMU.
>>
> 
> It's been a while (late 2013 if my memory serves me right).
> 
> Allowing KVM_MAX_VCPUS does sound reasonable then, yes.  But if you're
> testing on arm64 it probably won't work because you can't use
> architected timers with a userland gic and I don't think we emulate an
> external timer for the virt board on arm64.

Right, I just learned today that KVM does not even initialize on arm64
if there is no VGIC support available.
In fact I spent this morning to find a good solution for the issue that
with the above changes in the VGICv3-only case KVM_CREATE_DEVICE(GICV2)
is rightfully rejected, but KVM_CREATE_IRQCHIP still succeeds (though
the guest hangs later of course). Keep you posted on that progress...

Cheers,
Andre.