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Date: Fri, 20 Nov 2015 13:26:42 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] PCI: minor performance optimization
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To: Cao jin <caoj.fnst@cn.fujitsu.com>
Cc: qemu-devel@nongnu.org

On Fri, Nov 20, 2015 at 07:04:07PM +0800, Cao jin wrote:
> 
> 
> On 11/20/2015 06:45 PM, Michael S. Tsirkin wrote:
> >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.
> 
> OK
> 
> >
> >>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?
> >
> 
> For tiny performance improvement via less loop. take pcie express
> capability(60 bytes at most) for example, it may loop 60 times, now we just
> need 15 times, a quarter of before.

But who cares? This is not a data path operation.

> >>
> >>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
> >.
> >
> 
> -- 
> Yours Sincerely,
> 
> Cao Jin