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

From: Xiaoyao Li <[email-masked]>
Date: Wed, 30 Jul 2025 16:20:45 +0800

On 7/30/2025 6:54 AM, Sean Christopherson 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.

Based on above, I suppose the userspace_address is not NULL but some 
other separate userspace mapped memory.

> Signed-off-by: Sean Christopherson <[email-masked]>
> ---
>   .../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();

First glance at the name, it leads me to think about something of nested.

>   }