Re: [PATCH 09/20] KVM: selftests: Honor "stop" request in dirty ring test

Maxim Levitsky
2024-12-18T00:00:43Z

On Fri, 2024-12-13 at 17:07 -0800, Sean Christopherson wrote:
> Now that the vCPU doesn't dirty every page on the first iteration for
> architectures that support the dirty ring, honor vcpu_stop in the dirty
> ring's vCPU worker, i.e. stop when the main thread says "stop". This will
> allow plumbing vcpu_stop into the guest so that the vCPU doesn't need to
> periodically exit to userspace just to see if it should stop.

This is very misleading - by the very nature of this test it all runs in userspace,
so every time KVM_RUN ioctl exits, it is by definition an userspace VM exit.

>
> Add a comment explaining that marking all pages as dirty is problematic
> for the dirty ring, as it results in the guest getting stuck on "ring
> full". This could be addressed by adding a GUEST_SYNC() in that initial
> loop, but it's not clear how that would interact with s390's behavior.

I think that this commit description should be reworked to state that s390
doesn't support dirty ring currently so the test doesn't introduce a regression.

>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> tools/testing/selftests/kvm/dirty_log_test.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
> index 55a385499434..8d31e275a23d 100644
> --- a/tools/testing/selftests/kvm/dirty_log_test.c
> +++ b/tools/testing/selftests/kvm/dirty_log_test.c
> @@ -387,8 +387,7 @@ static void dirty_ring_after_vcpu_run(struct kvm_vcpu *vcpu)
>
> /* A ucall-sync or ring-full event is allowed */
> if (get_ucall(vcpu, NULL) == UCALL_SYNC) {
> - /* We should allow this to continue */
> - ;
> + vcpu_handle_sync_stop();
> } else if (run->exit_reason == KVM_EXIT_DIRTY_RING_FULL) {
> /* Update the flag first before pause */
> WRITE_ONCE(dirty_ring_vcpu_ring_full, true);
> @@ -697,6 +696,15 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> #ifdef __s390x__
> /* Align to 1M (segment size) */
> guest_test_phys_mem = align_down(guest_test_phys_mem, 1 << 20);
> +
> + /*
> + * The workaround in guest_code() to write all pages prior to the first
> + * iteration isn't compatible with the dirty ring, as the dirty ring
> + * support relies on the vCPU to actually stop when vcpu_stop is set so
> + * that the vCPU doesn't hang waiting for the dirty ring to be emptied.
> + */
> + TEST_ASSERT(host_log_mode != LOG_MODE_DIRTY_RING,
> + "Test needs to be updated to support s390 dirty ring");

This not clear either, the message makes me think that s390 does support dirty ring.
The comment above should state stat since s390 doesn't support dirty ring,
this is fine, and when/if the support is added,then the test will need to be updated.

> #endif
>
> pr_info("guest physical test memory offset: 0x%lx\n", guest_test_phys_mem);

Best regards,
Maxim Levitsky