Re: [PATCH v9 09/13] KVM: selftests: set KVM_MEM_GUEST_MEMFD in vm_mem_add() if guest_memfd != -1

[Ackerley Tng <ackerleytng@google.com>]

"Kalyazin, Nikita" <kalyazin@amazon.co.uk> writes:

> From: Patrick Roy <patrick.roy@linux.dev>
>
> Have vm_mem_add() always set KVM_MEM_GUEST_MEMFD in the memslot flags if
> a guest_memfd is passed in as an argument. This eliminates the
> possibility where a guest_memfd instance is passed to vm_mem_add(), but
> it ends up being ignored because the flags argument does not specify
> KVM_MEM_GUEST_MEMFD at the same time.
>
> This makes it easy to support more scenarios in which no vm_mem_add() is
> not passed a guest_memfd instance, but is expected to allocate one.
> Currently, this only happens if guest_memfd == -1 but flags &
> KVM_MEM_GUEST_MEMFD != 0, but later vm_mem_add() will gain support for
> loading the test code itself into guest_memfd (via
> GUEST_MEMFD_FLAG_MMAP) if requested via a special
> vm_mem_backing_src_type, at which point having to make sure the src_type
> and flags are in-sync becomes cumbersome.
>
> Signed-off-by: Patrick Roy <patrick.roy@linux.dev>
> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
> ---
>  tools/testing/selftests/kvm/lib/kvm_util.c | 24 +++++++++++++---------
>  1 file changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index 8279b6ced8d2..56ddbca91850 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -1057,21 +1057,25 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>
>  	region->backing_src_type = src_type;
>
> -	if (flags & KVM_MEM_GUEST_MEMFD) {
> -		if (guest_memfd < 0) {
> +	if (guest_memfd < 0) {
> +		if (flags & KVM_MEM_GUEST_MEMFD) {
>  			uint32_t guest_memfd_flags = 0;
>  			TEST_ASSERT(!guest_memfd_offset,
>  				    "Offset must be zero when creating new guest_memfd");
>  			guest_memfd = vm_create_guest_memfd(vm, mem_size, guest_memfd_flags);
> -		} else {
> -			/*
> -			 * Install a unique fd for each memslot so that the fd
> -			 * can be closed when the region is deleted without
> -			 * needing to track if the fd is owned by the framework
> -			 * or by the caller.
> -			 */
> -			guest_memfd = kvm_dup(guest_memfd);
>  		}
> +	} else {
> +		/*
> +		 * Install a unique fd for each memslot so that the fd
> +		 * can be closed when the region is deleted without
> +		 * needing to track if the fd is owned by the framework
> +		 * or by the caller.
> +		 */
> +		guest_memfd = kvm_dup(guest_memfd);
> +	}
> +
> +	if (guest_memfd > 0) {

Might 0 turn out to be a valid return from dup() for a guest_memfd?

> +		flags |= KVM_MEM_GUEST_MEMFD;
>
>  		region->region.guest_memfd = guest_memfd;
>  		region->region.guest_memfd_offset = guest_memfd_offset;

Refactoring vm_mem_add() (/* FIXME: This thing needs to be ripped apart
and rewritten. */) should probably be a separate patch series, but I'd
like to take this opportunity to ask: Sean, what do you have in mind for
the rewritten version?

Would it be something like struct vm_shape, where there are default
mem_shapes, and the shapes get validated and then passed to
vm_mem_add()?

> --
> 2.50.1