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SubjectRe: [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@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/
Plain text mails only, please http://www.expita.com/nomime.html
Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html



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