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

From: Xiao Guangrong
Date: Fri Mar 25 2016 - 10:08:13 EST

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))

+ idx = parents->idx[level];
clear_unsync_child_bit(sp, idx);

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. :)