Re: [PATCH v6 1/3] KVM: selftests: implement random number generation for guest code

[Sean Christopherson <seanjc@google.com>]

On Tue, Oct 11, 2022, Colton Lewis wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > On Mon, Sep 12, 2022, Colton Lewis wrote:
> > > Implement random number generation for guest code to randomize parts
> > > of the test, making it less predictable and a more accurate reflection
> > > of reality.
> 
> > > Create a -r argument to specify a random seed. If no argument is
> > > provided, the seed defaults to 0. The random seed is set with
> > > perf_test_set_random_seed() and must be set before guest_code runs to
> > > apply.
> 
> > > The random number generator chosen is the Park-Miller Linear
> > > Congruential Generator, a fancy name for a basic and well-understood
> > > random number generator entirely sufficient for this purpose. Each
> > > vCPU calculates its own seed by adding its index to the seed provided.
> 
> > Why not grab the kernel's pseudo-RNG from prandom_seed_state() +
> > prandom_u32_state()?
> 
> The guest is effectively a minimal kernel running in a VM that doesn't
> have access to this, correct?

Oh, I didn't mean link to the kernel code, I meant "why not copy+paste the kernel
code?".  In other words, why select a different RNG implementation than what the
kernel uses?  In general, selftests and tools try to follow the kernel code, even
when copy+pasting, as that avoids questions like "why does the kernel do X but
selftests do Y?".  The copy+paste does sometimes lead to maintenance pain, e.g. if
the copied code has a bug that the kernel then fixes, but that seems unlikely to
happen in this case.