linux-kernel - Re: [PATCH 04/22] KVM: selftests: Compute number of extra pages needed in mmu_stress

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Message-ID: <CADrL8HW6fFuFTm1wuW9UC4kr+rmRK4MqrU=bQEWram4xo9JBOw@mail.gmail.com>
Date: Thu, 5 Sep 2024 17:03:36 -0700
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 04/22] KVM: selftests: Compute number of extra pages
 needed in mmu_stress_test

On Fri, Aug 9, 2024 at 12:43 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Create mmu_stress_tests's VM with the correct number of extra pages needed
> to map all of memory in the guest.  The bug hasn't been noticed before as
> the test currently runs only on x86, which maps guest memory with 1GiB
> pages, i.e. doesn't need much memory in the guest for page tables.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  tools/testing/selftests/kvm/mmu_stress_test.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/kvm/mmu_stress_test.c b/tools/testing/selftests/kvm/mmu_stress_test.c
> index 847da23ec1b1..5467b12f5903 100644
> --- a/tools/testing/selftests/kvm/mmu_stress_test.c
> +++ b/tools/testing/selftests/kvm/mmu_stress_test.c
> @@ -209,7 +209,13 @@ int main(int argc, char *argv[])
>         vcpus = malloc(nr_vcpus * sizeof(*vcpus));
>         TEST_ASSERT(vcpus, "Failed to allocate vCPU array");
>
> -       vm = vm_create_with_vcpus(nr_vcpus, guest_code, vcpus);
> +       vm = __vm_create_with_vcpus(VM_SHAPE_DEFAULT, nr_vcpus,
> +#ifdef __x86_64__
> +                                   max_mem / SZ_1G,
> +#else
> +                                   max_mem / vm_guest_mode_params[VM_MODE_DEFAULT].page_size,
> +#endif
> +                                   guest_code, vcpus);

Hmm... I'm trying to square this change with the logic in
vm_nr_pages_required(). That logic seems to be doing what you want
(though it always assumes small mappings IIUC).

So it seems like there's something else that's not being accounted
for? (Also without the extra pages, how does this test actually fail?)