linux-kernel - Re: [PATCH 04/12] KVM: SEV: do not use list_replace

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Message-ID: <YaVT638kTtgF64/i@google.com>
Date:   Mon, 29 Nov 2021 22:27:55 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pgonda@...gle.com
Subject: Re: [PATCH 04/12] KVM: SEV: do not use list_replace_init on an empty
 list

On Mon, Nov 22, 2021, Paolo Bonzini wrote:
> list_replace_init cannot be used if the source is an empty list,
> because "new->next->prev = new" will overwrite "old->next":
> 
> 				new				old
> 				prev = new, next = new		prev = old, next = old
> new->next = old->next		prev = new, next = old		prev = old, next = old
> new->next->prev = new		prev = new, next = old		prev = old, next = new
> new->prev = old->prev		prev = old, next = old		prev = old, next = old
> new->next->prev = new		prev = old, next = old		prev = new, next = new
> 
> The desired outcome instead would be to leave both old and new the same
> as they were (two empty circular lists).  Use list_cut_before, which
> already has the necessary check and is documented to discard the
> previous contents of the list that will hold the result.
> 
> Fixes: b56639318bb2 ("KVM: SEV: Add support for SEV intra host migration")
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/svm/sev.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 21ac0a5de4e0..75955beb3770 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1613,8 +1613,7 @@ static void sev_migrate_from(struct kvm_sev_info *dst,
>  	src->handle = 0;
>  	src->pages_locked = 0;
>  
> -	INIT_LIST_HEAD(&dst->regions_list);
> -	list_replace_init(&src->regions_list, &dst->regions_list);
> +	list_cut_before(&dst->regions_list, &src->regions_list, &src->regions_list);

Yeesh, that is tricky.  A list_move_all() helper in list.h to do 

	list_cut_before(dst, src, src);

would be nice.

Reviewed-by: Sean Christopherson <seanjc@...gle.com>

>  }
>  
>  static int sev_es_migrate_from(struct kvm *dst, struct kvm *src)
> -- 
> 2.27.0
> 
>