Re: [PATCH 4/5] KVM: add __kvm_request_needs_mb

Paolo Bonzini
2017-02-23T11:01:20Z

On 16/02/2017 17:04, Radim Krčmář wrote:
> A macro to optimize requests that do not need a memory barrier because
> they have no dependencies. An architecture can implement a function
> that says which requests do not need memory barriers when handling them.
>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

I would leave this for a separate series, otherwise looks nice (though I
was skeptical at first ;)).

Paolo

> ---
> include/linux/kvm_host.h | 41 +++++++++++++++++++++++++++++++++++++----
> virt/kvm/kvm_main.c | 3 ++-
> 2 files changed, 39 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index d899473859d3..2cc438685af8 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1097,8 +1097,8 @@ static inline int kvm_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
> * 2) remote request with no data (= kick)
> * 3) remote request with data (= kick + mb)
> *
> - * TODO: the API is inconsistent -- a request doesn't call kvm_vcpu_kick(), but
> - * forces smp_wmb() for all requests.
> + * TODO: the API does not distinguish local and remote requests -- remote
> + * should contain kvm_vcpu_kick().
> */
>
> static inline void __kvm_request_set(unsigned req, struct kvm_vcpu *vcpu)
> @@ -1106,6 +1106,37 @@ static inline void __kvm_request_set(unsigned req, struct kvm_vcpu *vcpu)
> set_bit(req, &vcpu->requests);
> }
>
> +/*
> + * __kvm_request_needs_mb is used to improve performance, so it should have no
> + * runtime overhead.
> + */
> +static inline bool __kvm_request_needs_mb(int req)
> +{
> + /*
> + * This barrier lets callers avoid the following pattern:
> + * if (__kvm_request_needs_mb(req))
> + * ...
> + * else
> + * barrier();
> + */
> + barrier();
> +
> + if (!__builtin_constant_p(req))
> + return true;
> +
> +#ifdef kvm_arch_request_needs_mb
> + /*
> + * GCC optimizes pure kvm_arch_request_needs_mb() with a constant input
> + * into a contant, but __builtin_constant_p() is not so clever, so we
> + * cannot ensure that with:
> + * BUILD_BUG_ON(!__builtin_constant_p(kvm_arch_request_needs_mb(req)));
> + */
> + return kvm_arch_request_needs_mb(req);
> +#else
> + return true;
> +#endif
> +}
> +
> static inline void kvm_request_set(unsigned req, struct kvm_vcpu *vcpu)
> {
> /*
> @@ -1113,7 +1144,8 @@ static inline void kvm_request_set(unsigned req, struct kvm_vcpu *vcpu)
> * kvm_request_test_and_clear's caller.
> * Paired with the smp_mb__after_atomic in kvm_request_test_and_clear.
> */
> - smp_wmb();
> + if (__kvm_request_needs_mb(req))
> + smp_wmb();
> __kvm_request_set(req, vcpu);
> }
>
> @@ -1137,7 +1169,8 @@ static inline bool kvm_request_test_and_clear(unsigned req, struct kvm_vcpu *vcp
> * kvm_request_test_and_clear's caller.
> * Paired with the smp_wmb in kvm_request_set.
> */
> - smp_mb__after_atomic();
> + if (__kvm_request_needs_mb(req))
> + smp_mb__after_atomic();
> return true;
> } else {
> return false;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 2250920ec965..ced3e4cb1df0 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -179,7 +179,8 @@ bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req)
> me = get_cpu();
>
> /* Paired with the smp_mb__after_atomic in kvm_request_test_and_clear. */
> - smp_wmb();
> + if (__kvm_request_needs_mb(req))
> + smp_wmb();
>
> kvm_for_each_vcpu(i, vcpu, kvm) {
> __kvm_request_set(req, vcpu);
>