Re: [PATCH 4/4] selftests: KVM: use dirty logging to check if page stats work correctly

[Mingwei Zhang <mizhang@google.com>]

On Mon, Mar 21, 2022, Ben Gardon wrote:
> On Sun, Mar 20, 2022 at 5:26 PM Mingwei Zhang <mizhang@google.com> wrote:
> >
> > When dirty logging is enabled, KVM splits the all hugepage mapping in
> > NPT/EPT into the smallest 4K size. This property could be used to check if
> 
> Note this is only true if eager page splitting is enabled. It would be
> more accurate to say:
> "While dirty logging is enabled, KVM will re-map any accessed page in
> NPT/EPT at 4K."
> 
> > the page stats metrics work properly in KVM mmu. At the same time, this
> > logic might be used the other way around: using page stats to verify if
> > dirty logging really splits all huge pages. Moreover, when dirty logging is
> 
> It might be worth having a follow up commit which checks if eager
> splitting is enabled and changes the assertions accordingly.

So eager splitting is still pending for review, right? But yes, I can
add one after the feature get merged.

> 
> > disabled, KVM zaps corresponding SPTEs and we could check whether the large
> > pages come back when guest touches the pages again.
> >
> > So add page stats checking in dirty logging performance selftest. In
> > particular, add checks in three locations:
> >  - just after vm is created;
> >  - after populating memory into vm but before enabling dirty logging;
> >  - just after turning on dirty logging.
> 
> Note a key stage here is after dirty logging is enabled, and then the
> VM touches all the memory in the data region.
> I believe that's the point at which you're making the assertion that
> all mappings are 4k currently, which is the right place if eager
> splitting is not enabled.

Oh, sorry. This one should be after dirty logging is done, not 'just
after turning on dirty logging'. Will update it.

> 
> >  - after one final iteration after turning off dirty logging.
> >
> > Tested using commands:
> >  - ./dirty_log_perf_test -s anonymous_hugetlb_1gb
> >  - ./dirty_log_perf_test -s anonymous_thp
> >
> > Cc: Sean Christopherson <seanjc@google.com>
> > Cc: David Matlack <dmatlack@google.com>
> > Cc: Jing Zhang <jingzhangos@google.com>
> > Cc: Peter Xu <peterx@redhat.com>
> >
> > Suggested-by: Ben Gardon <bgorden@google.com>
> > Signed-off-by: Mingwei Zhang <mizhang@google.com>
> > ---
> >  .../selftests/kvm/dirty_log_perf_test.c       | 52 +++++++++++++++++++
> >  1 file changed, 52 insertions(+)
> >
> > diff --git a/tools/testing/selftests/kvm/dirty_log_perf_test.c b/tools/testing/selftests/kvm/dirty_log_perf_test.c
> > index 1954b964d1cf..ab0457d91658 100644
> > --- a/tools/testing/selftests/kvm/dirty_log_perf_test.c
> > +++ b/tools/testing/selftests/kvm/dirty_log_perf_test.c
> > @@ -19,6 +19,10 @@
> >  #include "perf_test_util.h"
> >  #include "guest_modes.h"
> >
> > +#ifdef __x86_64__
> > +#include "processor.h"
> > +#endif
> > +
> >  /* How many host loops to run by default (one KVM_GET_DIRTY_LOG for each loop)*/
> >  #define TEST_HOST_LOOP_N               2UL
> >
> > @@ -185,6 +189,14 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >                                  p->slots, p->backing_src,
> >                                  p->partition_vcpu_memory_access);
> >
> > +#ifdef __x86_64__
> > +       TEST_ASSERT(vm_get_single_stat(vm, "pages_4k") == 0,
> > +                   "4K page is non zero");
> > +       TEST_ASSERT(vm_get_single_stat(vm, "pages_2m") == 0,
> > +                   "2M page is non zero");
> > +       TEST_ASSERT(vm_get_single_stat(vm, "pages_1g") == 0,
> > +                   "1G page is non zero");
> > +#endif
> >         perf_test_set_wr_fract(vm, p->wr_fract);
> >
> >         guest_num_pages = (nr_vcpus * guest_percpu_mem_size) >> vm_get_page_shift(vm);
> > @@ -222,6 +234,16 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >         pr_info("Populate memory time: %ld.%.9lds\n",
> >                 ts_diff.tv_sec, ts_diff.tv_nsec);
> >
> > +#ifdef __x86_64__
> > +       TEST_ASSERT(vm_get_single_stat(vm, "pages_4k") != 0,
> > +                   "4K page is zero");
> > +       if (p->backing_src == VM_MEM_SRC_ANONYMOUS_THP)
> 
> This should also handle 2M hugetlb memory.
> I think there might be a library function to translate backing src
> type to page size too, which could make this check cleaner.

Ack.
> 
> > +               TEST_ASSERT(vm_get_single_stat(vm, "pages_2m") != 0,
> > +                           "2M page is zero");
> > +       if (p->backing_src == VM_MEM_SRC_ANONYMOUS_HUGETLB_1GB)
> > +               TEST_ASSERT(vm_get_single_stat(vm, "pages_1g") != 0,
> > +                           "1G page is zero");
> > +#endif
> >         /* Enable dirty logging */
> >         clock_gettime(CLOCK_MONOTONIC, &start);
> >         enable_dirty_logging(vm, p->slots);
> > @@ -267,6 +289,14 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >                                 iteration, ts_diff.tv_sec, ts_diff.tv_nsec);
> >                 }
> >         }
> > +#ifdef __x86_64__
> > +       TEST_ASSERT(vm_get_single_stat(vm, "pages_4k") != 0,
> > +                   "4K page is zero after dirty logging");
> > +       TEST_ASSERT(vm_get_single_stat(vm, "pages_2m") == 0,
> > +                   "2M page is non-zero after dirty logging");
> > +       TEST_ASSERT(vm_get_single_stat(vm, "pages_1g") == 0,
> > +                   "1G page is non-zero after dirty logging");
> > +#endif
> 
> Note this is after dirty logging has been enabled, AND all pages in
> the data region have been written by the guest.
> 
> >
> >         /* Disable dirty logging */
> >         clock_gettime(CLOCK_MONOTONIC, &start);
> > @@ -275,6 +305,28 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> >         pr_info("Disabling dirty logging time: %ld.%.9lds\n",
> >                 ts_diff.tv_sec, ts_diff.tv_nsec);
> >
> > +#ifdef __x86_64__
> > +       /*
> > +        * Increment iteration to run the vcpus again to verify if huge pages
> > +        * come back.
> > +        */
> > +       iteration++;
> > +       pr_info("Starting the final iteration to verify page stats\n");
> > +
> > +       for (vcpu_id = 0; vcpu_id < nr_vcpus; vcpu_id++) {
> > +               while (READ_ONCE(vcpu_last_completed_iteration[vcpu_id])
> > +                      != iteration)
> > +                       ;
> > +       }
> 
> We might as well do this on all archs. Even without the stats, it at
> least validates that disabling dirty logging doesn't break the VM.
> 
Ack.
> > +
> > +       if (p->backing_src == VM_MEM_SRC_ANONYMOUS_THP)
> > +               TEST_ASSERT(vm_get_single_stat(vm, "pages_2m") != 0,
> > +                           "2M page is zero");
> > +       if (p->backing_src == VM_MEM_SRC_ANONYMOUS_HUGETLB_1GB)
> > +               TEST_ASSERT(vm_get_single_stat(vm, "pages_1g") != 0,
> > +                           "1G page is zero");
> > +#endif
> > +
> >         /* Tell the vcpu thread to quit */
> >         host_quit = true;
> >         perf_test_join_vcpu_threads(nr_vcpus);
> > --
> > 2.35.1.894.gb6a874cedc-goog
> >