Subject: Re: [PATCH 3/4] KVM: MMU: reduce the size of mmu_page_path
To: Xiao Guangrong <guangrong.xiao@linux.intel.com>
References: <1458911978-19430-1-git-send-email-guangrong.xiao@linux.intel.com>
 <1458911978-19430-3-git-send-email-guangrong.xiao@linux.intel.com>
 <56F54103.6020508@redhat.com> <56F541C5.6090904@linux.intel.com>
Cc: gleb@kernel.org, mtosatti@redhat.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <56F54397.9050809@redhat.com>
Date: Fri, 25 Mar 2016 14:56:39 +0100
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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++;

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)...

Paolo