linux-kernel - Re: [PATCH V5 0/2] selftests: KVM: Add a test for eager page splitting

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Date:   Wed, 15 Mar 2023 12:22:20 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Peter Xu <peterx@...hat.com>,
        Vipin Sharma <vipinsh@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>
Subject: Re: [PATCH V5 0/2] selftests: KVM: Add a test for eager page splitting

On Wed, Mar 15, 2023, Paolo Bonzini wrote:
> On 3/15/23 13:24, Paolo Bonzini wrote:
> > On Tue, Mar 14, 2023 at 5:00 PM David Matlack <dmatlack@...gle.com> wrote:
> > > I wonder if pages are getting swapped, especially if running on a
> > > workstation. If so, mlock()ing all guest memory VMAs might be
> > > necessary to be able to assert exact page counts.
> > 
> > I don't think so, it's 100% reproducible and the machine is idle and
> > only accessed via network. Also has 64 GB of RAM. :)
> 
> It also reproduces on Intel with pml=0 and eptad=0; the reason is due
> to the different semantics of dirty bits for page-table pages on AMD
> and Intel.  Both AMD and eptad=0 Intel treat those as writes, therefore
> more pages are dropped before the repopulation phase when dirty logging
> is disabled.
> 
> The "missing" page had been included in the population phase because it
> hosts the page tables for vcpu_args, but repopulation does not need it.
> 
> This fixes it:
> 
> -------------------- 8< ---------------
> From: Paolo Bonzini <pbonzini@...hat.com>
> Subject: [PATCH] selftests: KVM: perform the same memory accesses on every memstress iteration
> 
> Perform the same memory accesses including the initialization steps
> that read from args and vcpu_args.  This ensures that the state of
> KVM's page tables is the same after every iteration, including the
> pages that host the guest page tables for args and vcpu_args.
> 
> This fixes a failure of dirty_log_page_splitting_test on AMD machines,
> as well as on Intel if PML and EPT A/D bits are both disabled.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> diff --git a/tools/testing/selftests/kvm/lib/memstress.c b/tools/testing/selftests/kvm/lib/memstress.c
> index 3632956c6bcf..8a429f4c86db 100644
> --- a/tools/testing/selftests/kvm/lib/memstress.c
> +++ b/tools/testing/selftests/kvm/lib/memstress.c
> @@ -56,15 +56,15 @@ void memstress_guest_code(uint32_t vcpu_idx)
>  	uint64_t page;
>  	int i;
> -	rand_state = new_guest_random_state(args->random_seed + vcpu_idx);
> +	while (true) {
> +		rand_state = new_guest_random_state(args->random_seed + vcpu_idx);

Doesn't this partially defeat the randomization that some tests like want?  E.g.
a test that wants to heavily randomize state will get the same pRNG for every
iteration.  Seems like we should have a knob to control whether or not each
iteration needs to be identical.