linux-kernel - Re: [PATCH v1 01/18] KVM: selftests/kvm_util: use array of pointers to maintain vcpus in kvm

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Message-ID: <Y1nHL5BUoWPqUtt9@google.com>
Date:   Wed, 26 Oct 2022 23:47:59 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     pbonzini@...hat.com, dmatlack@...gle.com, vipinsh@...gle.com,
        ajones@...tanamicro.com, eric.auger@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 01/18] KVM: selftests/kvm_util: use array of pointers
 to maintain vcpus in kvm_vm

On Mon, Oct 24, 2022, Wei Wang wrote:
> Each vcpu has an id associated with it and is intrinsically faster
> and easier to be referenced by indexing into an array with "vcpu->id",
> compared to using a list of vcpus in the current implementation. Change
> the vcpu list to an array of vcpu pointers. Users then don't need to
> allocate such a vcpu array on their own, and instead, they can reuse
> the one maintained in kvm_vm.
> 
> Signed-off-by: Wei Wang <wei.w.wang@...el.com>
> ---
>  .../testing/selftests/kvm/include/kvm_util.h  |  4 +++
>  .../selftests/kvm/include/kvm_util_base.h     |  3 +-
>  tools/testing/selftests/kvm/lib/kvm_util.c    | 34 ++++++-------------
>  tools/testing/selftests/kvm/lib/x86_64/vmx.c  |  2 +-
>  4 files changed, 17 insertions(+), 26 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/include/kvm_util.h b/tools/testing/selftests/kvm/include/kvm_util.h
> index c9286811a4cb..5d5c8968fb06 100644
> --- a/tools/testing/selftests/kvm/include/kvm_util.h
> +++ b/tools/testing/selftests/kvm/include/kvm_util.h
> @@ -10,4 +10,8 @@
>  #include "kvm_util_base.h"
>  #include "ucall_common.h"
>  
> +#define vm_iterate_over_vcpus(vm, vcpu, i)				\

vm_for_each_vcpu() is more aligned with existing KVM terminology.

> +	for (i = 0, vcpu = vm->vcpus[0];				\
> +		vcpu && i < KVM_MAX_VCPUS; vcpu = vm->vcpus[++i])

I hate pointer arithmetic more than most people, but in this case it avoids the
need to pass in 'i', which in turn cuts down on boilerplate and churn.

>  #endif /* SELFTEST_KVM_UTIL_H */
> diff --git a/tools/testing/selftests/kvm/include/kvm_util_base.h b/tools/testing/selftests/kvm/include/kvm_util_base.h
> index e42a09cd24a0..c90a9609b853 100644
> --- a/tools/testing/selftests/kvm/include/kvm_util_base.h
> +++ b/tools/testing/selftests/kvm/include/kvm_util_base.h
> @@ -45,7 +45,6 @@ struct userspace_mem_region {
>  };
>  
>  struct kvm_vcpu {
> -	struct list_head list;
>  	uint32_t id;
>  	int fd;
>  	struct kvm_vm *vm;
> @@ -75,7 +74,6 @@ struct kvm_vm {
>  	unsigned int pa_bits;
>  	unsigned int va_bits;
>  	uint64_t max_gfn;
> -	struct list_head vcpus;
>  	struct userspace_mem_regions regions;
>  	struct sparsebit *vpages_valid;
>  	struct sparsebit *vpages_mapped;
> @@ -92,6 +90,7 @@ struct kvm_vm {
>  	int stats_fd;
>  	struct kvm_stats_header stats_header;
>  	struct kvm_stats_desc *stats_desc;
> +	struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];

We can dynamically allocate the array without too much trouble, though I'm not
sure it's worth shaving the few KiB of memory.  For __vm_create(), the number of
vCPUs is known when the VM is created.  For vm_create_barebones(), we could do
the simple thing of allocating KVM_MAX_VCPU.

> @@ -534,6 +533,10 @@ __weak void vcpu_arch_free(struct kvm_vcpu *vcpu)
>  static void vm_vcpu_rm(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
>  {
>  	int ret;
> +	uint32_t vcpu_id = vcpu->id;
> +
> +	TEST_ASSERT(!!vm->vcpus[vcpu_id], "vCPU%d wasn't added\n", vcpu_id);

This is unecessary, there's one caller and it's iterating over the array of vCPUs.