[PATCH] KVM: x86: MMU: Initialize force_pt_level before calling mapping_level()

[Takuya Yoshikawa]

Commit fd1369021878 ("KVM: x86: MMU: Move mapping_level_dirty_bitmap()
call in mapping_level()") forgot to initialize force_pt_level to false
in FNAME(page_fault)() before calling mapping_level() like
nonpaging_map() does.  This can sometimes result in forcing page table
level mapping unnecessarily.

Fix this and move the first *force_pt_level check in mapping_level()
before kvm_vcpu_gfn_to_memslot() call to make it a bit clearer that
the variable must be initialized before mapping_level() gets called.

This change can also avoid calling kvm_vcpu_gfn_to_memslot() when
!check_hugepage_cache_consistency() check in tdp_page_fault() forces
page table level mapping.

Signed-off-by: Takuya Yoshikawa 
---
 arch/x86/kvm/mmu.c | 7 ---
 arch/x86/kvm/paging_tmpl.h | 2 +-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index dd2a7c6..7d85bca 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -886,10 +886,11 @@ static int mapping_level(struct kvm_vcpu *vcpu, gfn_t 
large_gfn,
int host_level, level, max_level;
struct kvm_memory_slot *slot;
 
-   slot = kvm_vcpu_gfn_to_memslot(vcpu, large_gfn);
+   if (unlikely(*force_pt_level))
+   return PT_PAGE_TABLE_LEVEL;
 
-   if (likely(!*force_pt_level))
-   *force_pt_level = !memslot_valid_for_gpte(slot, true);
+   slot = kvm_vcpu_gfn_to_memslot(vcpu, large_gfn);
+   *force_pt_level = !memslot_valid_for_gpte(slot, true);
if (unlikely(*force_pt_level))
return PT_PAGE_TABLE_LEVEL;
 
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index bf39d0f..b41faa9 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -698,7 +698,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t 
addr, u32 error_code,
int r;
pfn_t pfn;
int level = PT_PAGE_TABLE_LEVEL;
-   bool force_pt_level;
+   bool force_pt_level = false;
unsigned long mmu_seq;
bool map_writable, is_self_change_mapping;
 
-- 
2.1.0

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Re: [PATCH] KVM: x86: MMU: Initialize force_pt_level before calling mapping_level()

[Paolo Bonzini]

On 19/10/2015 08:13, Takuya Yoshikawa wrote:
> Commit fd1369021878 ("KVM: x86: MMU: Move mapping_level_dirty_bitmap()
> call in mapping_level()") forgot to initialize force_pt_level to false
> in FNAME(page_fault)() before calling mapping_level() like
> nonpaging_map() does.  This can sometimes result in forcing page table
> level mapping unnecessarily.
> 
> Fix this and move the first *force_pt_level check in mapping_level()
> before kvm_vcpu_gfn_to_memslot() call to make it a bit clearer that
> the variable must be initialized before mapping_level() gets called.
> 
> This change can also avoid calling kvm_vcpu_gfn_to_memslot() when
> !check_hugepage_cache_consistency() check in tdp_page_fault() forces
> page table level mapping.
> 
> Signed-off-by: Takuya Yoshikawa 
> ---
>  arch/x86/kvm/mmu.c | 7 ---
>  arch/x86/kvm/paging_tmpl.h | 2 +-
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index dd2a7c6..7d85bca 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -886,10 +886,11 @@ static int mapping_level(struct kvm_vcpu *vcpu, gfn_t 
> large_gfn,
>   int host_level, level, max_level;
>   struct kvm_memory_slot *slot;
>  
> - slot = kvm_vcpu_gfn_to_memslot(vcpu, large_gfn);
> + if (unlikely(*force_pt_level))
> + return PT_PAGE_TABLE_LEVEL;
>  
> - if (likely(!*force_pt_level))
> - *force_pt_level = !memslot_valid_for_gpte(slot, true);
> + slot = kvm_vcpu_gfn_to_memslot(vcpu, large_gfn);
> + *force_pt_level = !memslot_valid_for_gpte(slot, true);
>   if (unlikely(*force_pt_level))
>   return PT_PAGE_TABLE_LEVEL;
>  
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index bf39d0f..b41faa9 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -698,7 +698,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t 
> addr, u32 error_code,
>   int r;
>   pfn_t pfn;
>   int level = PT_PAGE_TABLE_LEVEL;
> - bool force_pt_level;
> + bool force_pt_level = false;
>   unsigned long mmu_seq;
>   bool map_writable, is_self_change_mapping;
>  
> 

Looks good, thanks.

Paolo
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