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



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