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

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Date:	Mon, 1 Nov 2010 19:42:04 +0100 (CET)
From:	Jesper Juhl <jj@...osbits.net>
To:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
cc:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Avi Kivity <avi@...ranet.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Yaniv Kamay <yaniv@...ranet.com>,
	Amit Shah <amit.shah@...ranet.com>,
	Ben-Ami Yassour <benami@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM x86: remove memset, use vzalloc and don't assign
 the same value to a variable twice

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@...osbits.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@...osbits.net>             http://www.chaosbits.net/
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