Re: [PATCH v3 4/6] KVM: Check for empty mask of harvested dirty ring entries in caller

[Binbin Wu <binbin.wu@linux.intel.com>]

On 5/17/2025 5:35 AM, Sean Christopherson wrote:
> When resetting a dirty ring, explicitly check that there is work to be
> done before calling kvm_reset_dirty_gfn(), e.g. if no harvested entries
> are found and/or on the loop's first iteration, and delete the extremely
> misleading comment "This is only needed to make compilers happy".  KVM
> absolutely relies on mask to be zero-initialized, i.e. the comment is an
> outright lie.  Furthermore, the compiler is right to complain that KVM is
> calling a function with uninitialized data, as there are no guarantees
> the implementation details of kvm_reset_dirty_gfn() will be visible to
> kvm_dirty_ring_reset().
>
> While the flaw could be fixed by simply deleting (or rewording) the
> comment, and duplicating the check is unfortunate, checking mask in the
> caller will allow for additional cleanups.
>
> Opportunistically drop the zero-initialization of cur_slot and cur_offset.
> If a bug were introduced where either the slot or offset was consumed
> before mask is set to a non-zero value, then it is highly desirable for
> the compiler (or some other sanitizer) to yell.
>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Yan Zhao <yan.y.zhao@intel.com>
> Cc: Maxim Levitsky <mlevitsk@redhat.com>
> Cc: Binbin Wu <binbin.wu@linux.intel.com>
> Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
> Reviewed-by: James Houghton <jthoughton@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

> ---
>   virt/kvm/dirty_ring.c | 44 ++++++++++++++++++++++++++++++++++---------
>   1 file changed, 35 insertions(+), 9 deletions(-)
>
> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> index 97cca0c02fd1..84c75483a089 100644
> --- a/virt/kvm/dirty_ring.c
> +++ b/virt/kvm/dirty_ring.c
> @@ -55,9 +55,6 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
>   	struct kvm_memory_slot *memslot;
>   	int as_id, id;
>   
> -	if (!mask)
> -		return;
> -
>   	as_id = slot >> 16;
>   	id = (u16)slot;
>   
> @@ -108,15 +105,24 @@ static inline bool kvm_dirty_gfn_harvested(struct kvm_dirty_gfn *gfn)
>   int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
>   			 int *nr_entries_reset)
>   {
> +	/*
> +	 * To minimize mmu_lock contention, batch resets for harvested entries
> +	 * whose gfns are in the same slot, and are within N frame numbers of
> +	 * each other, where N is the number of bits in an unsigned long.  For
> +	 * simplicity, process the current set of entries when the next entry
> +	 * can't be included in the batch.
> +	 *
> +	 * Track the current batch slot, the gfn offset into the slot for the
> +	 * batch, and the bitmask of gfns that need to be reset (relative to
> +	 * offset).  Note, the offset may be adjusted backwards, e.g. so that
> +	 * a sequence of gfns X, X-1, ... X-N can be batched.
> +	 */
>   	u32 cur_slot, next_slot;
>   	u64 cur_offset, next_offset;
> -	unsigned long mask;
> +	unsigned long mask = 0;
>   	struct kvm_dirty_gfn *entry;
>   	bool first_round = true;
>   
> -	/* This is only needed to make compilers happy */
> -	cur_slot = cur_offset = mask = 0;
> -
>   	while (likely((*nr_entries_reset) < INT_MAX)) {
>   		if (signal_pending(current))
>   			return -EINTR;
> @@ -164,14 +170,34 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
>   				continue;
>   			}
>   		}
> -		kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> +
> +		/*
> +		 * Reset the slot for all the harvested entries that have been
> +		 * gathered, but not yet fully processed.
> +		 */
> +		if (mask)
> +			kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> +
> +		/*
> +		 * The current slot was reset or this is the first harvested
> +		 * entry, (re)initialize the metadata.
> +		 */
>   		cur_slot = next_slot;
>   		cur_offset = next_offset;
>   		mask = 1;
>   		first_round = false;
>   	}
>   
> -	kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> +	/*
> +	 * Perform a final reset if there are harvested entries that haven't
> +	 * been processed, which is guaranteed if at least one harvested was
> +	 * found.  The loop only performs a reset when the "next" entry can't
> +	 * be batched with the "current" entry(s), and that reset processes the
> +	 * _current_ entry(s); i.e. the last harvested entry, a.k.a. next, will
> +	 * always be left pending.
> +	 */
> +	if (mask)
> +		kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
>   
>   	/*
>   	 * The request KVM_REQ_DIRTY_RING_SOFT_FULL will be cleared