Re: [PATCH v3 2/9] kvm: Prepare core VM structs and helpers for LUO support

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Missing `__rcu` annotation for the `vm_file` field in `struct kvm`.
--

commit 742b05343deb7763672dc99dce96c4773cc18720
Author: Tarun Sahu <tarunsahu@google.com>

kvm: Prepare core VM structs and helpers for LUO support

This commit introduces core infrastructure to support VM preservation with
LUO. It refactors VM file creation into a unified kvm_create_vm_file()
helper and tracks a weak reference to the backing file in struct kvm to
enable reverse file resolution.

> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
[ ... ]
> @@ -874,6 +874,18 @@ struct kvm {
>  #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
>  	/* Protected by slots_lock (for writes) and RCU (for reads) */
>  	struct xarray mem_attr_array;
> +#endif
> +#ifdef CONFIG_LIVEUPDATE_GUEST_MEMFD
> +	/*
> +	 * Weak reference to the VFS file backing this KVM instance. Stored
> +	 * without incrementing the file refcount to prevent a circular lifetime
> +	 * dependency (since file->private_data already pins this struct kvm).
> +	 * Used exclusively to resolve the file pointer back from struct kvm.
> +	 *
> +	 * Written/cleared via rcu_assign_pointer() and read locklessly under
> +	 * RCU (e.g. via get_file_active() to prevent ABA races).
> +	 */
> +	struct file *vm_file;

[Severity: Low]
Should this pointer be annotated with __rcu?

It is documented as being read locklessly under RCU and is updated using
rcu_assign_pointer() below. Without the annotation, sparse static analysis
will likely generate warnings about incorrect types and different address
spaces when the pointer is updated.

> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
[ ... ]
> +struct file *kvm_create_vm_file(unsigned long type, const char *fdname)
> +{
[ ... ]
> +#ifdef CONFIG_LIVEUPDATE_GUEST_MEMFD
> +	/*
> +	 * Weak reference to the file (without get_file()) to prevent a circular
> +	 * dependency. Safe because the file's release path clears this pointer
> +	 * and drops its reference to the VM.
> +	 *
> +	 * Written via rcu_assign_pointer() because the pointer can be read
> +	 * locklessly under RCU (e.g., in kvm_gmem_luo_preserve() via
> +	 * get_file_active() to prevent lockless ABA races).
> +	 */
> +	rcu_assign_pointer(kvm->vm_file, file);

[Severity: Low]
Since kvm->vm_file is updated here in kvm_create_vm_file() using
rcu_assign_pointer(), does the member in struct kvm require the __rcu
modifier to prevent sparse warnings?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260622184851.2309827-1-tarunsahu@google.com?part=2