Sean Christopherson <seanjc@xxxxxxxxxx> writes:
On Mon, Sep 12, 2022, Colton Lewis wrote:
diff --git a/tools/testing/selftests/kvm/include/test_util.h
b/tools/testing/selftests/kvm/include/test_util.h
index 99e0dcdc923f..2dd286bcf46f 100644
--- a/tools/testing/selftests/kvm/include/test_util.h
+++ b/tools/testing/selftests/kvm/include/test_util.h
@@ -143,4 +143,6 @@ static inline void *align_ptr_up(void *x, size_t
size)
return (void *)align_up((unsigned long)x, size);
}
+void guest_random(uint32_t *seed);
This is a weird and unintuitive API. The in/out param exposes the gory
details
of the pseudo-RNG to the caller, and makes it cumbersome to use, e.g. to
create
a 64-bit number or to consume the result in a conditional.
To explain my reasoning:
It's simple because there is exactly one way to use it and it's short so
anyone can understand how the function works at a glance. It's similar
to the API used by other thread-safe RNGs like rand_r and random_r. They
also use in/out parameters. That's the only way to buy thread
safety. Callers would also have to manage their own state in your
example with an in/out parameter if they want thread safety.
I disagree the details are gory. You put in a number and get a new
number. It's common knowledge PRNGs work this way. I understand you are
thinking about ease of future extensions, but this strikes me as
premature abstraction. Additional APIs can always be added later for the
fancy stuff without modifying the callers that don't need it.
I agree returning the value could make it easier to use as part of
expressions, but is it that important?
It's also not inherently guest-specific, or even KVM specific. We should
consider
landing this in common selftests code so that others can use it and even
expand on
it. E.g. in a previous life, I worked with a tool that implemented all
sorts of
random number magic that provided APIs to get random bools with 1->99
probabilty,
random numbers along Guassian curves, bounded numbers, etc.
People who need random numbers outside the guest should use stdlib.h. No
need to reimplement a full random library. The point of this
implementation was to do the simplest thing that could provide random
numbers to the guest.
