Re: [dpdk-dev] [PATCH v2 1/4] igb_uio: add wc option

["Rafał Kozik" <rk@semihalf.com>]

2018-06-28 16:32 GMT+02:00 Ferruh Yigit <ferruh.yigit@intel.com>:
> On 6/28/2018 2:15 PM, Rafal Kozik wrote:
>> Write combining (WC) increases NIC performance by making better
>> utilization of PCI bus, but cannot be use by all PMDs.
>>
>> To get internal_addr memory need to be mapped. But as memory could not be
>> mapped twice: with and without WC, it should be skipped for WC. [1]
>>
>> To do not spoil other drivers that potentially could use internal_addr,
>> parameter wc_activate adds possibility to skip it for those PMDs,
>> that do not use it.
>>
>> [1] https://www.kernel.org/doc/ols/2008/ols2008v2-pages-135-144.pdf
>>       section 5.3 and 5.4
>
> Hi Rafal,
>
> Thank you for more information but I have a few more question:
>
> - What do you mean "But as memory could not be mapped twice: with and without WC"?
>
> ioremap() maps the physical address for kernel usage, and via uio we are mapping
> it to userspace, do you mean these two?
>
> - "internal_addr" is should be for kernel sage not for DPDK drivers which are in
> the userspace, why it is a concern for us?
>
> - What happens if you don't update this code at all? Won't you able to map
> device address into userspace?
> I tested adding RTE_PCI_DRV_WC_ACTIVATE to i40e, on top of your patch, and able
> to map without igb_uio update.
> I am not able to understand need of the modification.
>

Hello Ferruh,

I was not precisely. Memory could be mapped multiple time,
but cannot be mapped with and without WC support simultaneously.
When not setting wc_activate memory mapping work, but silently
fall-back to non prefetchable mode.

I perform measurements of writing speed.
When parameter wc_activate was set I get 4.81 GB/s.
Without this parameter result was 0.07 GB/s.
Code used for testing is located here:
gist.github.com/semihalf-kozik-rafal/327208cd52a2fac2d12250028becf9b3

Best regards,
Rafal

>>
>> Signed-off-by: Rafal Kozik <rk@semihalf.com>
>> Acked-by: Bruce Richardson <bruce.richardson@intel.com>
>> ---
>>  kernel/linux/igb_uio/igb_uio.c | 17 ++++++++++++++---
>>  1 file changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/linux/igb_uio/igb_uio.c b/kernel/linux/igb_uio/igb_uio.c
>> index b3233f1..3382fb1 100644
>> --- a/kernel/linux/igb_uio/igb_uio.c
>> +++ b/kernel/linux/igb_uio/igb_uio.c
>> @@ -30,6 +30,7 @@ struct rte_uio_pci_dev {
>>       int refcnt;
>>  };
>>
>> +static int wc_activate;
>>  static char *intr_mode;
>>  static enum rte_intr_mode igbuio_intr_mode_preferred = RTE_INTR_MODE_MSIX;
>>  /* sriov sysfs */
>> @@ -375,9 +376,13 @@ igbuio_pci_setup_iomem(struct pci_dev *dev, struct uio_info *info,
>>       len = pci_resource_len(dev, pci_bar);
>>       if (addr == 0 || len == 0)
>>               return -1;
>> -     internal_addr = ioremap(addr, len);
>> -     if (internal_addr == NULL)
>> -             return -1;
>> +     if (wc_activate == 0) {
>> +             internal_addr = ioremap(addr, len);
>> +             if (internal_addr == NULL)
>> +                     return -1;
>> +     } else {
>> +             internal_addr = NULL;
>> +     }
>>       info->mem[n].name = name;
>>       info->mem[n].addr = addr;
>>       info->mem[n].internal_addr = internal_addr;
>> @@ -650,6 +655,12 @@ MODULE_PARM_DESC(intr_mode,
>>  "    " RTE_INTR_MODE_LEGACY_NAME "     Use Legacy interrupt\n"
>>  "\n");
>>
>> +module_param(wc_activate, int, 0);
>> +MODULE_PARM_DESC(wc_activate,
>> +"Activate support for write combining (WC) (default=0)\n"
>> +"    0 - disable\n"
>> +"    other - enable\n");
>> +
>>  MODULE_DESCRIPTION("UIO driver for Intel IGB PCI cards");
>>  MODULE_LICENSE("GPL");
>>  MODULE_AUTHOR("Intel Corporation");
>>
>