Re: [PATCH v1 2/4] KVM: selftests: Add an interface to read the data of named vcpu stat

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 21, 2024, Manali Shukla wrote:
> From: Manali Shukla <Manali.Shukla@amd.com>
> 
> The interface is used to read the data values of a specified vcpu stat
> from the currenly available binary stats interface.
> 
> Add a concatenation trickery to trigger compiler error if the stat
> doesn't exist, so that it is not possible to pass a per-VM stat into
> vcpu_get_stat().
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Manali Shukla <Manali.Shukla@amd.com>
> ---
>  .../testing/selftests/kvm/include/kvm_util.h  | 52 +++++++++++++++++++
>  .../kvm/include/x86_64/kvm_util_arch.h        | 36 +++++++++++++
>  tools/testing/selftests/kvm/lib/kvm_util.c    | 40 ++++++++++++++
>  3 files changed, 128 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/include/kvm_util.h b/tools/testing/selftests/kvm/include/kvm_util.h
> index bc7c242480d6..5dd3acf174f8 100644
> --- a/tools/testing/selftests/kvm/include/kvm_util.h
> +++ b/tools/testing/selftests/kvm/include/kvm_util.h
> @@ -531,6 +531,14 @@ void read_stat_data(int stats_fd, struct kvm_stats_header *header,
>  		    struct kvm_stats_desc *desc, uint64_t *data,
>  		    size_t max_elements);
>  
> +#define DEFINE_CHECK_STAT(type, stat)			\
> +static inline int check_##type##_##stat##_exists(void)	\
> +{							\
> +	return 1;					\
> +}							\
> +
> +#define STAT_EXISTS(type, stat) (check_##type##_##stat##_exists())

This is all unnecessary complicated.  To trigger a compilation error, the set
of knnown stats just needs to be defined as _something_ and then referenced.
There's no need for layers of macros and a function for each stat.  The fact that
a stat is defined is proof of its existence.

> +
>  void __vm_get_stat(struct kvm_vm *vm, const char *stat_name, uint64_t *data,
>  		   size_t max_elements);
>  
> @@ -542,6 +550,50 @@ static inline uint64_t vm_get_stat(struct kvm_vm *vm, const char *stat_name)
>  	return data;
>  }
>  
> +#define DEFINE_GENERIC_VCPU_STAT				\
> +	DEFINE_CHECK_STAT(vcpu, halt_successfull_poll)		\
> +	DEFINE_CHECK_STAT(vcpu, halt_attempted_poll)		\
> +	DEFINE_CHECK_STAT(vcpu, halt_poll_invalid)		\
> +	DEFINE_CHECK_STAT(vcpu, halt_wakeup)			\
> +	DEFINE_CHECK_STAT(vcpu, halt_poll_success_ns)		\
> +	DEFINE_CHECK_STAT(vcpu, halt_poll_fail_ns)		\
> +	DEFINE_CHECK_STAT(vcpu, halt_wait_ns)			\
> +	DEFINE_CHECK_STAT(vcpu, halt_poll_success_hist)		\
> +	DEFINE_CHECK_STAT(vcpu, halt_poll_fail_hist)		\
> +	DEFINE_CHECK_STAT(vcpu, halt_wait_hist)			\
> +	DEFINE_CHECK_STAT(vcpu, blocking)			\
> +
> +/*
> + * Define a default empty macro for architectures which do not specify
> + * arch specific vcpu stats
> + */
> +
> +#ifndef DEFINE_ARCH_VCPU_STAT
> +#define DEFINE_ARCH_VCPU_STAT
> +#endif
> +
> +DEFINE_GENERIC_VCPU_STAT

There's also no need to define macros in arch code just to expand them in common
code.  Add simple macros in kvm_util_types.h and this goes away.

> +DEFINE_ARCH_VCPU_STAT
> +
> +#undef DEFINE_CHECK_STAT
> +#undef DEFINE_GENERIC_VCPU_STAT
> +#undef DEFINE_ARCH_VCPU_STAT

> +void __vcpu_get_stat(struct kvm_vcpu *vcpu, const char *stat_name, uint64_t *data,
> +		   size_t max_elements);
> +
> +#define vcpu_get_stat(vcpu, stat_name)				\
> +({								\
> +	uint64_t data;						\
> +								\
> +	STAT_EXISTS(vcpu, stat_name);				\
> +	__vcpu_get_stat(vcpu, #stat_name, &data, 1);		\
> +	data;							\
> +})								\
> +
> +#undef DEFINE_CHECK_STAT
> +#undef DEFINE_GENERIC_VCPU_STAT
> +

...

> +void __vcpu_get_stat(struct kvm_vcpu *vcpu, const char *stat_name, uint64_t *data,
> +		   size_t max_elements)
> +{
> +	int vcpu_stats_fd;
> +	struct kvm_stats_header header;
> +	struct kvm_stats_desc *desc, *t_desc;
> +	size_t size_desc;
> +	int i;
> +
> +	vcpu_stats_fd = vcpu_get_stats_fd(vcpu);
> +	read_stats_header(vcpu_stats_fd, &header);
> +
> +	desc = read_stats_descriptors(vcpu_stats_fd, &header);
> +	size_desc = get_stats_descriptor_size(&header);
> +
> +	for (i = 0; i < header.num_desc; ++i) {
> +		t_desc = (void *)desc + (i * size_desc);
> +
> +		if (strcmp(t_desc->name, stat_name))
> +			continue;
> +
> +		read_stat_data(vcpu_stats_fd, &header, t_desc,
> +			       data, max_elements);
> +		break;
> +	}
> +}

This is copy-pasted nearly verbatim from the VM-scoped code.  It even has the
same bugs (doesn't assert the stat exists), along with new bugs (leaks the fd
and header).

It takes a bit of work, but not _that_ much work, to genericize the VM-scoped
infrastructure and reuse it for vCPU-scoped stats.