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

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Date:   Mon, 6 Sep 2021 06:33:35 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        ehabkost@...hat.com, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v2 5/6] kvm: allocate vcpu pointer array separately

On 03.09.21 16:41, Marc Zyngier wrote:
> On Fri, 03 Sep 2021 14:08:06 +0100,
> Juergen Gross <jgross@...e.com> 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@...e.com>
>> ---
>> 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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