On Mon, Mar 27, 2023, Colton Lewis wrote:
> diff --git a/tools/testing/selftests/kvm/include/aarch64/processor.h b/tools/testing/selftests/kvm/include/aarch64/processor.h
> index f65e491763e0..d441f485e9c6 100644
> --- a/tools/testing/selftests/kvm/include/aarch64/processor.h
> +++ b/tools/testing/selftests/kvm/include/aarch64/processor.h
> @@ -219,4 +219,14 @@ uint32_t guest_get_vcpuid(void);
> uint64_t cycles_read(void);
> uint64_t cycles_to_ns(struct kvm_vcpu *vcpu, uint64_t cycles);
>
> +#define MEASURE_CYCLES(x) \
> + ({ \
> + uint64_t start; \
> + start = cycles_read(); \
> + isb(); \
Would it make sense to put the necessary barriers inside the cycles_read() (or
whatever we end up calling it)? Or does that not make sense on ARM?
> + x; \
> + dsb(nsh); \
> + cycles_read() - start; \
> + })
> +
> #endif /* SELFTEST_KVM_PROCESSOR_H */
...
> diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
> index 5d977f95d5f5..7352e02db4ee 100644
> --- a/tools/testing/selftests/kvm/include/x86_64/processor.h
> +++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
> @@ -1137,4 +1137,14 @@ void virt_map_level(struct kvm_vm *vm, uint64_t vaddr, uint64_t paddr,
> uint64_t cycles_read(void);
> uint64_t cycles_to_ns(struct kvm_vcpu *vcpu, uint64_t cycles);
>
> +#define MEASURE_CYCLES(x) \
> + ({ \
> + uint64_t start; \
> + start = cycles_read(); \
> + asm volatile("mfence"); \
This is incorrect as placing the barrier after the RDTSC allows the RDTSC to be
executed before earlier loads, e.g. could measure memory accesses from whatever
was before MEASURE_CYCLES(). And per the kernel's rdtsc_ordered(), it sounds like
RDTSC can only be hoisted before prior loads, i.e. will be ordered with respect
to future loads and stores.
> + x; \
> + asm volatile("mfence"); \
> + cycles_read() - start; \
> + })
