linux-kernel - Re: [PATCH v17 24/24] KVM: selftests: Add guest_memfd testcase to fault-in on !mmap()'d memory

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Message-ID: <CA+EHjTx7V=bHZ3HTN+gK+MzNhwDehiqQXG2r4AYmMKDufYGOww@mail.gmail.com>
Date: Wed, 30 Jul 2025 16:51:47 +0100
From: Fuad Tabba <tabba@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, kvm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>, 
	Gavin Shan <gshan@...hat.com>, Shivank Garg <shivankg@....com>, Vlastimil Babka <vbabka@...e.cz>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, David Hildenbrand <david@...hat.com>, 
	Ackerley Tng <ackerleytng@...gle.com>, Tao Chan <chentao@...inos.cn>, 
	James Houghton <jthoughton@...gle.com>
Subject: Re: [PATCH v17 24/24] KVM: selftests: Add guest_memfd testcase to
 fault-in on !mmap()'d memory

On Tue, 29 Jul 2025 at 23:56, Sean Christopherson <seanjc@...gle.com> wrote:
>
> Add a guest_memfd testcase to verify that a vCPU can fault-in guest_memfd
> memory that supports mmap(), but that is not currently mapped into host
> userspace and/or has a userspace address (in the memslot) that points at
> something other than the target guest_memfd range.  Mapping guest_memfd
> memory into the guest is supposed to operate completely independently from
> any userspace mappings.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Fuad Tabba <tabba@...gle.com>
Tested-by: Fuad Tabba <tabba@...gle.com>

Cheers,
/fuad



> ---
>  .../testing/selftests/kvm/guest_memfd_test.c  | 64 +++++++++++++++++++
>  1 file changed, 64 insertions(+)
>
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 088053d5f0f5..b86bf89a71e0 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -13,6 +13,7 @@
>
>  #include <linux/bitmap.h>
>  #include <linux/falloc.h>
> +#include <linux/sizes.h>
>  #include <setjmp.h>
>  #include <signal.h>
>  #include <sys/mman.h>
> @@ -21,6 +22,7 @@
>
>  #include "kvm_util.h"
>  #include "test_util.h"
> +#include "ucall_common.h"
>
>  static void test_file_read_write(int fd)
>  {
> @@ -298,6 +300,66 @@ static void test_guest_memfd(unsigned long vm_type)
>         kvm_vm_free(vm);
>  }
>
> +static void guest_code(uint8_t *mem, uint64_t size)
> +{
> +       size_t i;
> +
> +       for (i = 0; i < size; i++)
> +               __GUEST_ASSERT(mem[i] == 0xaa,
> +                              "Guest expected 0xaa at offset %lu, got 0x%x", i, mem[i]);
> +
> +       memset(mem, 0xff, size);
> +       GUEST_DONE();
> +}
> +
> +static void test_guest_memfd_guest(void)
> +{
> +       /*
> +        * Skip the first 4gb and slot0.  slot0 maps <1gb and is used to back
> +        * the guest's code, stack, and page tables, and low memory contains
> +        * the PCI hole and other MMIO regions that need to be avoided.
> +        */
> +       const uint64_t gpa = SZ_4G;
> +       const int slot = 1;
> +
> +       struct kvm_vcpu *vcpu;
> +       struct kvm_vm *vm;
> +       uint8_t *mem;
> +       size_t size;
> +       int fd, i;
> +
> +       if (!kvm_has_cap(KVM_CAP_GUEST_MEMFD_MMAP))
> +               return;
> +
> +       vm = __vm_create_shape_with_one_vcpu(VM_SHAPE_DEFAULT, &vcpu, 1, guest_code);
> +
> +       TEST_ASSERT(vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP),
> +                   "Default VM type should always support guest_memfd mmap()");
> +
> +       size = vm->page_size;
> +       fd = vm_create_guest_memfd(vm, size, GUEST_MEMFD_FLAG_MMAP);
> +       vm_set_user_memory_region2(vm, slot, KVM_MEM_GUEST_MEMFD, gpa, size, NULL, fd, 0);
> +
> +       mem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +       TEST_ASSERT(mem != MAP_FAILED, "mmap() on guest_memfd failed");
> +       memset(mem, 0xaa, size);
> +       munmap(mem, size);
> +
> +       virt_pg_map(vm, gpa, gpa);
> +       vcpu_args_set(vcpu, 2, gpa, size);
> +       vcpu_run(vcpu);
> +
> +       TEST_ASSERT_EQ(get_ucall(vcpu, NULL), UCALL_DONE);
> +
> +       mem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> +       TEST_ASSERT(mem != MAP_FAILED, "mmap() on guest_memfd failed");
> +       for (i = 0; i < size; i++)
> +               TEST_ASSERT_EQ(mem[i], 0xff);
> +
> +       close(fd);
> +       kvm_vm_free(vm);
> +}
> +
>  int main(int argc, char *argv[])
>  {
>         unsigned long vm_types, vm_type;
> @@ -314,4 +376,6 @@ int main(int argc, char *argv[])
>
>         for_each_set_bit(vm_type, &vm_types, BITS_PER_TYPE(vm_types))
>                 test_guest_memfd(vm_type);
> +
> +       test_guest_memfd_guest();
>  }
> --
> 2.50.1.552.g942d659e1b-goog
>