Re: [Qemu-devel] [PATCH] PCI: minor performance optimization

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 20, 2015 at 06:45:01PM +0800, Cao jin wrote:
> 1. Do param check in pci_add_capability2(), as it is a public API.

Separate patch pls.

> 2. As spec says, each capability must be DWORD aligned, so an optimization can
>    be done via Loop Unrolling.

Why do we want to optimize it?

> 
> Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
> ---
>  hw/pci/pci.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> index 168b9cc..1e99603 100644
> --- a/hw/pci/pci.c
> +++ b/hw/pci/pci.c
> @@ -1924,13 +1924,15 @@ PCIDevice *pci_create_simple(PCIBus *bus, int devfn, const char *name)
>  static uint8_t pci_find_space(PCIDevice *pdev, uint8_t size)
>  {
>      int offset = PCI_CONFIG_HEADER_SIZE;
> -    int i;
> -    for (i = PCI_CONFIG_HEADER_SIZE; i < PCI_CONFIG_SPACE_SIZE; ++i) {
> +    int i = PCI_CONFIG_HEADER_SIZE;;
> +
> +    for (; i < PCI_CONFIG_SPACE_SIZE; i = i + 4) {
>          if (pdev->used[i])
> -            offset = i + 1;
> -        else if (i - offset + 1 == size)
> +            offset = i + 4;
> +        else if (i - offset >= size)
>              return offset;
>      }
> +
>      return 0;
>  }
>  
> @@ -2144,6 +2146,8 @@ int pci_add_capability2(PCIDevice *pdev, uint8_t cap_id,
>      uint8_t *config;
>      int i, overlapping_cap;
>  
> +    assert(size > 0);
> +
>      if (!offset) {
>          offset = pci_find_space(pdev, size);
>          if (!offset) {
> -- 
> 2.1.0