linux-kernel - Re: [PATCH v2 4/5] KVM: Check for empty mask of harvested dirty ring entries in caller

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Message-ID: <458bb0e1-ed50-449c-b884-a825eb09b7fe@linux.intel.com>
Date: Tue, 13 May 2025 17:17:52 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Peter Xu <peterx@...hat.com>,
 Yan Zhao <yan.y.zhao@...el.com>, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH v2 4/5] KVM: Check for empty mask of harvested dirty ring
 entries in caller



On 5/8/2025 10:10 PM, 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.
>
> Opportunisticaly drop the zero-initialization of cur_slot and cur_offset.

Opportunisticaly -> Opportunistically


> 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@...hat.com>
> Cc: Yan Zhao <yan.y.zhao@...el.com>
> Cc: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   virt/kvm/dirty_ring.c | 44 ++++++++++++++++++++++++++++++++++---------
>   1 file changed, 35 insertions(+), 9 deletions(-)
>
[...]
>   
> -	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 "current" the entry(s), and that reset processes the
"current" the entry(s) -> the "current" entry(s) ?

> +	 * _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