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

[Paolo Bonzini <pbonzini@redhat.com>]

On 3/15/23 20:22, Sean Christopherson wrote:
> 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@google.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@redhat.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@redhat.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.

Yes, this wasn't really a full patch, just to prove what the bug is.

One possibility to avoid adding a new knob is to do something like:

unsigned iteration = 0;
rand_state = new_guest_random_state(args->random_seed
	+ vcpu_idx + iteration++);

Paolo