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

[Paolo Bonzini]

On 3/15/23 13:24, Paolo Bonzini wrote:
> On Tue, Mar 14, 2023 at 5:00 PM David Matlack <dmatlack@xxxxxxxxxx> 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@xxxxxxxxxx>
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@xxxxxxxxxx>

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);
 
-	gva = vcpu_args->gva;
-	pages = vcpu_args->pages;
+		gva = vcpu_args->gva;
+		pages = vcpu_args->pages;
 
-	/* Make sure vCPU args data structure is not corrupt. */
-	GUEST_ASSERT(vcpu_args->vcpu_idx == vcpu_idx);
+		/* Make sure vCPU args data structure is not corrupt. */
+		GUEST_ASSERT(vcpu_args->vcpu_idx == vcpu_idx);
 
-	while (true) {
 		for (i = 0; i < pages; i++) {
 			if (args->random_access)
 				page = guest_random_u32(&rand_state) % pages;

Paolo