Re: [PATCH 3/3] KVM: selftests: Print summary stats of memory latency distribution

[Sean Christopherson <seanjc@google.com>]

On Tue, Nov 15, 2022, Colton Lewis wrote:
> Print summary stats of the memory latency distribution in
> nanoseconds. For every iteration, this prints the minimum, the
> maximum, and the 50th, 90th, and 99th percentiles.
> 
> Stats are calculated by sorting the samples taken from all vcpus and
> picking from the index corresponding with each percentile.
> 
> The conversion to nanoseconds needs the frequency of the Intel
> timestamp counter, which is estimated by reading the counter before
> and after sleeping for 1 second. This is not a pretty trick, but it
> also exists in vmx_nested_tsc_scaling_test.c

This test shouldn't need to guesstimate the frequency, just use a VM-scoped
KVM_GET_TSC_KHZ, which will provide KVM's default TSC frequency, i.e. the host
frequency.  For hardware with a constant TSC, which is everything modern, that
will be as accurate as we can get.  For hardware without a constant TSC, well, buy
new hardware :-)

vmx_nested_tsc_scaling_test.c does the weird sleep() behavior because it's trying
to validate that the guest TSC counts at the correct rate, i.e. it is validating
KVM_GET_TSC_KHZ to some extent, and so obviously doesn't fully trust its result.
   
For tests that just want to measure time, there's no reason not to trust KVM.