[PATCH] KVM x86: remove memset, use vzalloc and don't assign the same value to a variable twice

[PATCH] KVM x86: remove memset, use vzalloc and don't assign the same value to a variable twice

[Jesper Juhl]

Hi,

We can improve kvm_vm_ioctl_get_dirty_log() slightly by using vzalloc() 
rather than first allocating and then manually zero the memory with 
memset(). Also, while I was looking at this I noticed that we assign 
-ENOMEM to the 'r' variable twice even though none of the code inbetween 
the two assignments can change 'r', so I removed the second assignment.

Patch has been compile tested only.

Please consider merging and please CC me on all replies as I'm not 
subscribed to the kvm mailing list.


Signed-off-by: Jesper Juhl <jj@chaosbits.net>
---
 x86.c |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2288ad8..29f9c0a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3174,12 +3174,10 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		spin_unlock(&kvm->mmu_lock);
 
 		r = -ENOMEM;
-		dirty_bitmap = vmalloc(n);
+		dirty_bitmap = vzalloc(n);
 		if (!dirty_bitmap)
 			goto out;
-		memset(dirty_bitmap, 0, n);
 
-		r = -ENOMEM;
 		slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
 		if (!slots) {
 			vfree(dirty_bitmap);


-- 
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Re: [PATCH] KVM x86: remove memset, use vzalloc and don't assign the same value to a variable twice

[Takuya Yoshikawa]

(2010/10/31 3:28), Jesper Juhl wrote:
> Hi,
>
> We can improve kvm_vm_ioctl_get_dirty_log() slightly by using vzalloc()
> rather than first allocating and then manually zero the memory with
> memset(). Also, while I was looking at this I noticed that we assign

I personally prefer this new vzalloc() to vmalloc() + memset().

Just from my interest, is there real performance difference not just
the cleanup effect?  If so, we'd better do this for other places too.


> -ENOMEM to the 'r' variable twice even though none of the code inbetween
> the two assignments can change 'r', so I removed the second assignment.
>
> Patch has been compile tested only.
>
> Please consider merging and please CC me on all replies as I'm not
> subscribed to the kvm mailing list.
>
>
> Signed-off-by: Jesper Juhl<jj@chaosbits.net>
> ---
>   x86.c |    4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2288ad8..29f9c0a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3174,12 +3174,10 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
>   		spin_unlock(&kvm->mmu_lock);

This patch is not based on kvm.git, I guess.

>
>   		r = -ENOMEM;
> -		dirty_bitmap = vmalloc(n);
> +		dirty_bitmap = vzalloc(n);
>   		if (!dirty_bitmap)
>   			goto out;
> -		memset(dirty_bitmap, 0, n);
>
> -		r = -ENOMEM;

This one is here because this belongs to a different code block from the
previous one.  This keeps it easy to insert another codes in between these
two blocks.  The optimization will be done at compile time.
IIRC, I did like this based on Avi's advise.

   Takuya

>   		slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
>   		if (!slots) {
>   			vfree(dirty_bitmap);
>
>

Re: [PATCH] KVM x86: remove memset, use vzalloc and don't assign the same value to a variable twice

[Jesper Juhl]

On Mon, 1 Nov 2010, Takuya Yoshikawa wrote:

> (2010/10/31 3:28), Jesper Juhl wrote:
> > Hi,
> > 
> > We can improve kvm_vm_ioctl_get_dirty_log() slightly by using vzalloc()
> > rather than first allocating and then manually zero the memory with
> > memset(). Also, while I was looking at this I noticed that we assign
> 
> I personally prefer this new vzalloc() to vmalloc() + memset().
> 
> Just from my interest, is there real performance difference not just
> the cleanup effect?  If so, we'd better do this for other places too.
> 
There's definately a positive size impact for the generated object code 
and we save having to do the call to memset() and the cost of a vzalloc() 
call looks more or less the same as a call to vmalloc() to me.


<snip>
> 
> This patch is not based on kvm.git, I guess.
> 
Nope. It was generated against mainline git as of a couple of days ago. I 
can generate a version against kvm.git if you prefer.


> >   		r = -ENOMEM;
> > -		dirty_bitmap = vmalloc(n);
> > +		dirty_bitmap = vzalloc(n);
> >   		if (!dirty_bitmap)
> >   			goto out;
> > -		memset(dirty_bitmap, 0, n);
> > 
> > -		r = -ENOMEM;
> 
> This one is here because this belongs to a different code block from the
> previous one.  This keeps it easy to insert another codes in between these
> two blocks.  The optimization will be done at compile time.
> IIRC, I did like this based on Avi's advise.
> 
Fair enough, I've dropped that bit from the patch. Updated version below 
(against current mainline git).


Signed-off-by: Jesper Juhl <jj@chaosbits.net>
---
 x86.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2288ad8..624d4da 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3174,10 +3174,9 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		spin_unlock(&kvm->mmu_lock);
 
 		r = -ENOMEM;
-		dirty_bitmap = vmalloc(n);
+		dirty_bitmap = vzalloc(n);
 		if (!dirty_bitmap)
 			goto out;
-		memset(dirty_bitmap, 0, n);
 
 		r = -ENOMEM;
 		slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);



-- 
Jesper Juhl <jj@chaosbits.net>             http://www.chaosbits.net/
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Re: [PATCH] KVM x86: remove memset, use vzalloc and don't assign the same value to a variable twice

[Takuya Yoshikawa]

Hi Jesper, (dropped some addresses from Cc)

> > Jesper Juhl wrote:
> There's definately a positive size impact for the generated object code 
> and we save having to do the call to memset() and the cost of a vzalloc() 
> call looks more or less the same as a call to vmalloc() to me.

> > This patch is not based on kvm.git, I guess.
> > 
> Nope. It was generated against mainline git as of a couple of days ago. I 
> can generate a version against kvm.git if you prefer.

Sorry, I should have said more clearly.

kvm_vm_ioctl_get_dirty_log() has been changed since latest mainline kernel was released.
Furthermore, vmalloc() in it is to be removed soon, currently in kvm's next.

I have checked all vmalloc() + memset() in kvm, and there are three remaining.
See the patch below.
  - I have tested on x86.
  - I can test ppc later if needed, but this is so trivial that Alex will see
    no problem about this, probably.

So please write your Signed-off-by and ask Avi and Marcelo to apply.

Thanks,
  Takuya

===
Subject: [PATCH] KVM: replace vmalloc and memset with vzalloc

Let's use newly introduced vzalloc().

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Cc: Jesper Juhl <jj@chaosbits.net>
---
 arch/powerpc/kvm/book3s.c |    4 +---
 virt/kvm/kvm_main.c       |    9 ++-------
 2 files changed, 3 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
index e316847..badc983 100644
--- a/arch/powerpc/kvm/book3s.c
+++ b/arch/powerpc/kvm/book3s.c
@@ -1307,12 +1307,10 @@ struct kvm_vcpu *kvmppc_core_vcpu_create(struct kvm *kvm, unsigned int id)
 	int err = -ENOMEM;
 	unsigned long p;
 
-	vcpu_book3s = vmalloc(sizeof(struct kvmppc_vcpu_book3s));
+	vcpu_book3s = vzalloc(sizeof(struct kvmppc_vcpu_book3s));
 	if (!vcpu_book3s)
 		goto out;
 
-	memset(vcpu_book3s, 0, sizeof(struct kvmppc_vcpu_book3s));
-
 	vcpu_book3s->shadow_vcpu = (struct kvmppc_book3s_shadow_vcpu *)
 		kzalloc(sizeof(*vcpu_book3s->shadow_vcpu), GFP_KERNEL);
 	if (!vcpu_book3s->shadow_vcpu)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index dbe1d6a..bd44a81 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -606,13 +606,11 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	/* Allocate if a slot is being created */
 #ifndef CONFIG_S390
 	if (npages && !new.rmap) {
-		new.rmap = vmalloc(npages * sizeof(*new.rmap));
+		new.rmap = vzalloc(npages * sizeof(*new.rmap));
 
 		if (!new.rmap)
 			goto out_free;
 
-		memset(new.rmap, 0, npages * sizeof(*new.rmap));
-
 		new.user_alloc = user_alloc;
 		new.userspace_addr = mem->userspace_addr;
 	}
@@ -635,14 +633,11 @@ int __kvm_set_memory_region(struct kvm *kvm,
 			     >> KVM_HPAGE_GFN_SHIFT(level));
 		lpages -= base_gfn >> KVM_HPAGE_GFN_SHIFT(level);
 
-		new.lpage_info[i] = vmalloc(lpages * sizeof(*new.lpage_info[i]));
+		new.lpage_info[i] = vzalloc(lpages * sizeof(*new.lpage_info[i]));
 
 		if (!new.lpage_info[i])
 			goto out_free;
 
-		memset(new.lpage_info[i], 0,
-		       lpages * sizeof(*new.lpage_info[i]));
-
 		if (base_gfn & (KVM_PAGES_PER_HPAGE(level) - 1))
 			new.lpage_info[i][0].write_count = 1;
 		if ((base_gfn+npages) & (KVM_PAGES_PER_HPAGE(level) - 1))
-- 
1.7.0.4

Re: [PATCH] KVM x86: remove memset, use vzalloc and don't assign the same value to a variable twice

[Jesper Juhl]

On Tue, 2 Nov 2010, Takuya Yoshikawa wrote:

> Hi Jesper, (dropped some addresses from Cc)
> 
> > > Jesper Juhl wrote:
> > There's definately a positive size impact for the generated object code 
> > and we save having to do the call to memset() and the cost of a vzalloc() 
> > call looks more or less the same as a call to vmalloc() to me.
> 
> > > This patch is not based on kvm.git, I guess.
> > > 
> > Nope. It was generated against mainline git as of a couple of days ago. I 
> > can generate a version against kvm.git if you prefer.
> 
> Sorry, I should have said more clearly.
> 
> kvm_vm_ioctl_get_dirty_log() has been changed since latest mainline kernel was released.
> Furthermore, vmalloc() in it is to be removed soon, currently in kvm's next.
> 
> I have checked all vmalloc() + memset() in kvm, and there are three remaining.
> See the patch below.
>   - I have tested on x86.
>   - I can test ppc later if needed, but this is so trivial that Alex will see
>     no problem about this, probably.
> 
> So please write your Signed-off-by and ask Avi and Marcelo to apply.
> 
> Thanks,
>   Takuya
> 
> ===
> Subject: [PATCH] KVM: replace vmalloc and memset with vzalloc
> 
> Let's use newly introduced vzalloc().
> 
> Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>

Signed-off-by: Jesper-juhl <jj@chaosbits.net>


-- 
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Re: [PATCH] KVM x86: remove memset, use vzalloc and don't assign the same value to a variable twice

[Marcelo Tosatti]

On Tue, Nov 02, 2010 at 05:11:19AM +0100, Jesper Juhl wrote:
> On Tue, 2 Nov 2010, Takuya Yoshikawa wrote:
> 
> > Hi Jesper, (dropped some addresses from Cc)
> > 
> > > > Jesper Juhl wrote:
> > > There's definately a positive size impact for the generated object code 
> > > and we save having to do the call to memset() and the cost of a vzalloc() 
> > > call looks more or less the same as a call to vmalloc() to me.
> > 
> > > > This patch is not based on kvm.git, I guess.
> > > > 
> > > Nope. It was generated against mainline git as of a couple of days ago. I 
> > > can generate a version against kvm.git if you prefer.
> > 
> > Sorry, I should have said more clearly.
> > 
> > kvm_vm_ioctl_get_dirty_log() has been changed since latest mainline kernel was released.
> > Furthermore, vmalloc() in it is to be removed soon, currently in kvm's next.
> > 
> > I have checked all vmalloc() + memset() in kvm, and there are three remaining.
> > See the patch below.
> >   - I have tested on x86.
> >   - I can test ppc later if needed, but this is so trivial that Alex will see
> >     no problem about this, probably.
> > 
> > So please write your Signed-off-by and ask Avi and Marcelo to apply.
> > 
> > Thanks,
> >   Takuya
> > 
> > ===
> > Subject: [PATCH] KVM: replace vmalloc and memset with vzalloc
> > 
> > Let's use newly introduced vzalloc().
> > 
> > Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
> 
> Signed-off-by: Jesper-juhl <jj@chaosbits.net>

Applied, thanks.