Re: [PATCH v2 5/6] kvm: allocate vcpu pointer array separately

[Juergen Gross <jgross@xxxxxxxx>]

On 03.09.21 16:41, Marc Zyngier wrote:
> On Fri, 03 Sep 2021 14:08:06 +0100,
> Juergen Gross <jgross@xxxxxxxx> wrote:
> > Prepare support of very large vcpu numbers per guest by moving the
> > vcpu pointer array out of struct kvm.
> >
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> > V2:
> > - rebase to new kvm_arch_free_vm() implementation
> > ---
> >   arch/arm64/kvm/arm.c            | 21 +++++++++++++++++++--
> >   arch/x86/include/asm/kvm_host.h |  5 +----
> >   arch/x86/kvm/x86.c              | 18 ++++++++++++++++++
> >   include/linux/kvm_host.h        | 17 +++++++++++++++--
> >   4 files changed, 53 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 38fff5963d9f..8bb5caeba007 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -293,10 +293,27 @@ long kvm_arch_dev_ioctl(struct file *filp,
> >   
> >   struct kvm *kvm_arch_alloc_vm(void)
> >   {
> > +	struct kvm *kvm;
> > +
> > +	if (!has_vhe())
> > +		kvm = kzalloc(sizeof(struct kvm), GFP_KERNEL);
> > +	else
> > +		kvm = vzalloc(sizeof(struct kvm));
> > +
> > +	if (!kvm)
> > +		return NULL;
> > +
> >   	if (!has_vhe())
> > -		return kzalloc(sizeof(struct kvm), GFP_KERNEL);
> > +		kvm->vcpus = kcalloc(KVM_MAX_VCPUS, sizeof(void *), GFP_KERNEL);
> > +	else
> > +		kvm->vcpus = vzalloc(KVM_MAX_VCPUS * sizeof(void *));
> > +
> > +	if (!kvm->vcpus) {
> > +		kvm_arch_free_vm(kvm);
> > +		kvm = NULL;
> > +	}
> >   
> > -	return vzalloc(sizeof(struct kvm));
> > +	return kvm;
> >   }
> >   
> >   int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index f16fadfc030a..6c28d0800208 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1517,10 +1517,7 @@ static inline void kvm_ops_static_call_update(void)
> >   }
> >   
> >   #define __KVM_HAVE_ARCH_VM_ALLOC
> > -static inline struct kvm *kvm_arch_alloc_vm(void)
> > -{
> > -	return __vmalloc(kvm_x86_ops.vm_size, GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> > -}
> > +struct kvm *kvm_arch_alloc_vm(void);
> >   
> >   #define __KVM_HAVE_ARCH_VM_FREE
> >   void kvm_arch_free_vm(struct kvm *kvm);
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index cc552763f0e4..ff142b6dd00c 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -11126,6 +11126,24 @@ void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu)
> >   	static_call(kvm_x86_sched_in)(vcpu, cpu);
> >   }
> >   
> > +struct kvm *kvm_arch_alloc_vm(void)
> > +{
> > +	struct kvm *kvm;
> > +
> > +	kvm = __vmalloc(kvm_x86_ops.vm_size, GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> > +	if (!kvm)
> > +		return NULL;
> > +
> > +	kvm->vcpus = __vmalloc(KVM_MAX_VCPUS * sizeof(void *),
> > +			       GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> > +	if (!kvm->vcpus) {
> > +		vfree(kvm);
> > +		kvm = NULL;
> > +	}
> > +
> > +	return kvm;
> > +}
> > +
> >   void kvm_arch_free_vm(struct kvm *kvm)
> >   {
> >   	kfree(to_kvm_hv(kvm)->hv_pa_pg);
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index d75e9c2a00b1..9e2a5f1c6f54 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -536,7 +536,7 @@ struct kvm {
> >   	struct mutex slots_arch_lock;
> >   	struct mm_struct *mm; /* userspace tied to this vm */
> >   	struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
> > -	struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
> > +	struct kvm_vcpu **vcpus;
>
> At this stage, I really wonder why we are not using an xarray instead.
>
> I wrote this [1] a while ago, and nothing caught fire. It was also a
> net deletion of code...

Indeed, I'd prefer that solution!

Are you fine with me swapping my patch with yours in the series?


Juergen

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