Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

[Xu Yilun]

> +KVM_SET_USER_MEMORY_REGION2 is an extension to KVM_SET_USER_MEMORY_REGION that
> +allows mapping guest_memfd memory into a guest.  All fields shared with
> +KVM_SET_USER_MEMORY_REGION identically.  Userspace can set KVM_MEM_PRIVATE in
> +flags to have KVM bind the memory region to a given guest_memfd range of
> +[guest_memfd_offset, guest_memfd_offset + memory_size].  The target guest_memfd
                                                        ^
The range end should be exclusive, is it?

> +must point at a file created via KVM_CREATE_GUEST_MEMFD on the current VM, and
> +the target range must not be bound to any other memory region.  All standard
> +bounds checks apply (use common sense).
> +
>  ::
>  
>    struct kvm_userspace_memory_region2 {
> @@ -6087,9 +6096,24 @@ applied.
>  	__u64 guest_phys_addr;
>  	__u64 memory_size; /* bytes */
>  	__u64 userspace_addr; /* start of the userspace allocated memory */
> +  __u64 guest_memfd_offset;
> +	__u32 guest_memfd;
> +	__u32 pad1;
> +	__u64 pad2[14];
>    };
>  

[...]

> +static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
> +{
> +	const char *anon_name = "[kvm-gmem]";
> +	struct kvm_gmem *gmem;
> +	struct inode *inode;
> +	struct file *file;
> +	int fd, err;
> +
> +	fd = get_unused_fd_flags(0);
> +	if (fd < 0)
> +		return fd;
> +
> +	gmem = kzalloc(sizeof(*gmem), GFP_KERNEL);
> +	if (!gmem) {
> +		err = -ENOMEM;
> +		goto err_fd;
> +	}
> +
> +	/*
> +	 * Use the so called "secure" variant, which creates a unique inode
> +	 * instead of reusing a single inode.  Each guest_memfd instance needs
> +	 * its own inode to track the size, flags, etc.
> +	 */
> +	file = anon_inode_getfile_secure(anon_name, &kvm_gmem_fops, gmem,
> +					 O_RDWR, NULL);
> +	if (IS_ERR(file)) {
> +		err = PTR_ERR(file);
> +		goto err_gmem;
> +	}
> +
> +	file->f_flags |= O_LARGEFILE;
> +
> +	inode = file->f_inode;
> +	WARN_ON(file->f_mapping != inode->i_mapping);

Just curious, why should we check the mapping fields which is garanteed in
other subsystem?

> +
> +	inode->i_private = (void *)(unsigned long)flags;
> +	inode->i_op = &kvm_gmem_iops;
> +	inode->i_mapping->a_ops = &kvm_gmem_aops;
> +	inode->i_mode |= S_IFREG;
> +	inode->i_size = size;
> +	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
> +	mapping_set_unmovable(inode->i_mapping);
> +	/* Unmovable mappings are supposed to be marked unevictable as well. */
> +	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
> +
> +	kvm_get_kvm(kvm);
> +	gmem->kvm = kvm;
> +	xa_init(&gmem->bindings);
> +	list_add(&gmem->entry, &inode->i_mapping->private_list);
> +
> +	fd_install(fd, file);
> +	return fd;
> +
> +err_gmem:
> +	kfree(gmem);
> +err_fd:
> +	put_unused_fd(fd);
> +	return err;
> +}

[...]

> +int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
> +		  unsigned int fd, loff_t offset)
> +{
> +	loff_t size = slot->npages << PAGE_SHIFT;
> +	unsigned long start, end;
> +	struct kvm_gmem *gmem;
> +	struct inode *inode;
> +	struct file *file;
> +
> +	BUILD_BUG_ON(sizeof(gfn_t) != sizeof(slot->gmem.pgoff));
> +
> +	file = fget(fd);
> +	if (!file)
> +		return -EBADF;
> +
> +	if (file->f_op != &kvm_gmem_fops)
> +		goto err;
> +
> +	gmem = file->private_data;
> +	if (gmem->kvm != kvm)
> +		goto err;
> +
> +	inode = file_inode(file);
> +
> +	if (offset < 0 || !PAGE_ALIGNED(offset))
> +		return -EINVAL;

Should also "goto err" here.

> +
> +	if (offset + size > i_size_read(inode))
> +		goto err;
> +
> +	filemap_invalidate_lock(inode->i_mapping);
> +
> +	start = offset >> PAGE_SHIFT;
> +	end = start + slot->npages;
> +
> +	if (!xa_empty(&gmem->bindings) &&
> +	    xa_find(&gmem->bindings, &start, end - 1, XA_PRESENT)) {
> +		filemap_invalidate_unlock(inode->i_mapping);
> +		goto err;
> +	}
> +
> +	/*
> +	 * No synchronize_rcu() needed, any in-flight readers are guaranteed to
> +	 * be see either a NULL file or this new file, no need for them to go
> +	 * away.
> +	 */
> +	rcu_assign_pointer(slot->gmem.file, file);
> +	slot->gmem.pgoff = start;
> +
> +	xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
> +	filemap_invalidate_unlock(inode->i_mapping);
> +
> +	/*
> +	 * Drop the reference to the file, even on success.  The file pins KVM,
> +	 * not the other way 'round.  Active bindings are invalidated if the
                             ^
around?

Thanks,
Yilun

> +	 * file is closed before memslots are destroyed.
> +	 */
> +	fput(file);
> +	return 0;
> +
> +err:
> +	fput(file);
> +	return -EINVAL;
> +}