Re: [PATCH 3/4] KVM: MMU: reduce the size of mmu_page_path

[Xiao Guangrong <guangrong.xiao@linux.intel.com>]

On 03/25/2016 09:56 PM, Paolo Bonzini wrote:
>
>
> On 25/03/2016 14:48, Xiao Guangrong wrote:
>>>>
>>>
>>> This patch and the previous one are basically redoing commit
>>> 0a47cd85833e ("KVM: MMU: Fix ubsan warnings", 2016-03-04).  While you
>>> find your version easier to understand, I of course find mine easier.
>>>
>>> Rather than getting stuck in a ko fight, the solution is to stick with
>>> the code in KVM and add comments.  I'll give it a try...
>>
>> If you do not like this one, we can just make the .index is
>> [PT64_ROOT_LEVEL - 1] and keep the sentinel in .parents[], that little
>> change and nice code shape.
>
> I suppose you'd have something like this then:
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 70e95d097ef1..15e1735a2e3a 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1980,7 +1980,7 @@ static bool kvm_sync_pages(struct kvm_vcpu *vcpu, gfn_t gfn,
>
>   struct mmu_page_path {
>   	struct kvm_mmu_page *parent[PT64_ROOT_LEVEL];
> -	unsigned int idx[PT64_ROOT_LEVEL];
> +	unsigned int idx[PT64_ROOT_LEVEL-1];
>   };
>
>   #define for_each_sp(pvec, sp, parents, i)			\
> @@ -2037,13 +2037,14 @@ static void mmu_pages_clear_parents(struct mmu_page_path *parents)
>   {
>   	struct kvm_mmu_page *sp;
>   	unsigned int level = 0;
> +	unsigned int idx;
>
>   	do {
> -		unsigned int idx = parents->idx[level];
>   		sp = parents->parent[level];
> -		if (!sp)
> +		if (!sp || WARN_ON(level == PT64_ROOT_LEVEL-1))
>   			return;
>
> +		idx = parents->idx[level];
>   		WARN_ON(idx == INVALID_INDEX);
>   		clear_unsync_child_bit(sp, idx);
>   		level++;
>

Yes, exactly.

[ actually, we can keep mmu_pages_clear_parents() unchanged ]

> By making the arrays the same size, the effect of the sentinel seems
> clearer to me.  It doesn't seem worth 4 bytes (and strictly speaking
> those 4 bytes would be there anyway due to padding)...

The sentinel is NULL forever so it can not go to the inner loop anyway...

Okay, i am not strong opinion on it, it is not a big deal. Let's
happily drop it if you really dislike it. :)