linux-kernel - Re: [PATCH v4 3/7] KVM: mmu: Refactor memslot copy

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Message-ID: <YJrRN2S3EJI/S01o@google.com>
Date:   Tue, 11 May 2021 18:47:19 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Kai Huang <kai.huang@...el.com>,
        Keqian Zhu <zhukeqian1@...wei.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4 3/7] KVM: mmu: Refactor memslot copy

On Tue, May 11, 2021, Ben Gardon wrote:
> Factor out copying kvm_memslots from allocating the memory for new ones
> in preparation for adding a new lock to protect the arch-specific fields
> of the memslots.
> 
> No functional change intended.
> 
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
>  virt/kvm/kvm_main.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 6b4feb92dc79..9e106742b388 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1306,6 +1306,18 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
>  	return old_memslots;
>  }
>  
> +static size_t kvm_memslots_size(int slots)

Can we call this kvm_calc_memslots_size()?  This doesn't actually return the
true size of a given memslots instance since the allocated size may be greater
than the size computed by looking at used_slots.

> +{
> +	return sizeof(struct kvm_memslots) +
> +	       (sizeof(struct kvm_memory_slot) * slots);
> +}
> +
> +static void kvm_copy_memslots(struct kvm_memslots *from,
> +			      struct kvm_memslots *to)
> +{
> +	memcpy(to, from, kvm_memslots_size(from->used_slots));
> +}
> +
>  /*
>   * Note, at a minimum, the current number of used slots must be allocated, even
>   * when deleting a memslot, as we need a complete duplicate of the memslots for
> @@ -1315,19 +1327,16 @@ static struct kvm_memslots *kvm_dup_memslots(struct kvm_memslots *old,
>  					     enum kvm_mr_change change)
>  {
>  	struct kvm_memslots *slots;
> -	size_t old_size, new_size;
> -
> -	old_size = sizeof(struct kvm_memslots) +
> -		   (sizeof(struct kvm_memory_slot) * old->used_slots);
> +	size_t new_size;
>  
>  	if (change == KVM_MR_CREATE)
> -		new_size = old_size + sizeof(struct kvm_memory_slot);
> +		new_size = kvm_memslots_size(old->used_slots + 1);
>  	else
> -		new_size = old_size;
> +		new_size = kvm_memslots_size(old->used_slots);
>  
>  	slots = kvzalloc(new_size, GFP_KERNEL_ACCOUNT);
>  	if (likely(slots))
> -		memcpy(slots, old, old_size);
> +		kvm_copy_memslots(old, slots);
>  
>  	return slots;
>  }
> -- 
> 2.31.1.607.g51e8a6a459-goog
>