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

[Cao jin <caoj.fnst@cn.fujitsu.com>]

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.

>>
>> 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