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SubjectRe: [PATCH 3/4] KVM: MMU: reduce the size of mmu_page_path
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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. :)

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