Re: [PATCH v7 7/7] KVM: guest_memfd: Add a guest_memfd() flag to initialize it as shared

[Ackerley Tng <ackerleytng@google.com>]

Fuad Tabba <tabba@google.com> writes:

> Not all use cases require guest_memfd() to be shared with the host when
> first created. Add a new flag, GUEST_MEMFD_FLAG_INIT_SHARED, which when
> set on KVM_CREATE_GUEST_MEMFD initializes the memory as shared with the
> host, and therefore mappable by it. Otherwise, memory is private until
> explicitly shared by the guest with the host.
>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  Documentation/virt/kvm/api.rst                 |  4 ++++
>  include/uapi/linux/kvm.h                       |  1 +
>  tools/testing/selftests/kvm/guest_memfd_test.c |  7 +++++--
>  virt/kvm/guest_memfd.c                         | 12 ++++++++++++
>  4 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 2b52eb77e29c..a5496d7d323b 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6386,6 +6386,10 @@ most one mapping per page, i.e. binding multiple memory regions to a single
>  guest_memfd range is not allowed (any number of memory regions can be bound to
>  a single guest_memfd file, but the bound ranges must not overlap).
>  
> +If the capability KVM_CAP_GMEM_SHARED_MEM is supported, then the flags field
> +supports GUEST_MEMFD_FLAG_INIT_SHARED, which initializes the memory as shared
> +with the host, and thereby, mappable by it.
> +
>  See KVM_SET_USER_MEMORY_REGION2 for additional details.
>  
>  4.143 KVM_PRE_FAULT_MEMORY
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 117937a895da..22d7e33bf09c 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1566,6 +1566,7 @@ struct kvm_memory_attributes {
>  #define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
>  
>  #define KVM_CREATE_GUEST_MEMFD	_IOWR(KVMIO,  0xd4, struct kvm_create_guest_memfd)
> +#define GUEST_MEMFD_FLAG_INIT_SHARED		(1UL << 0)
>  
>  struct kvm_create_guest_memfd {
>  	__u64 size;
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 38c501e49e0e..4a7fcd6aa372 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -159,7 +159,7 @@ static void test_invalid_punch_hole(int fd, size_t page_size, size_t total_size)
>  static void test_create_guest_memfd_invalid(struct kvm_vm *vm)
>  {
>  	size_t page_size = getpagesize();
> -	uint64_t flag;
> +	uint64_t flag = BIT(0);
>  	size_t size;
>  	int fd;
>  
> @@ -170,7 +170,10 @@ static void test_create_guest_memfd_invalid(struct kvm_vm *vm)
>  			    size);
>  	}
>  
> -	for (flag = BIT(0); flag; flag <<= 1) {
> +	if (kvm_has_cap(KVM_CAP_GMEM_SHARED_MEM))
> +		flag = GUEST_MEMFD_FLAG_INIT_SHARED << 1;
> +
> +	for (; flag; flag <<= 1) {

This would end up shifting the GUEST_MEMFD_FLAG_INIT_SHARED flag for
each loop.

>  		fd = __vm_create_guest_memfd(vm, page_size, flag);
>  		TEST_ASSERT(fd == -1 && errno == EINVAL,
>  			    "guest_memfd() with flag '0x%lx' should fail with EINVAL",
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index eec9d5e09f09..32e149478b04 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -1069,6 +1069,15 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
>  		goto err_gmem;
>  	}
>  
> +	if (IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM) &&
> +	    (flags & GUEST_MEMFD_FLAG_INIT_SHARED)) {
> +		err = kvm_gmem_offset_range_set_shared(file_inode(file), 0, size >> PAGE_SHIFT);
> +		if (err) {
> +			fput(file);
> +			goto err_gmem;
> +		}
> +	}
> +
>  	kvm_get_kvm(kvm);
>  	gmem->kvm = kvm;
>  	xa_init(&gmem->bindings);
> @@ -1090,6 +1099,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
>  	u64 flags = args->flags;
>  	u64 valid_flags = 0;
>  
> +	if (IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM))
> +		valid_flags |= GUEST_MEMFD_FLAG_INIT_SHARED;
> +
>  	if (flags & ~valid_flags)
>  		return -EINVAL;