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

From: [email-masked] (Christoffer Dall)
Date: Tue, 6 Jan 2015 14:29:27 +0100

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.

-Christoffer