Re: [dpdk-dev] [PATCH] pci/uio: enable prefetchable resources mapping

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Feb 01, 2018 at 09:18:22AM +0800, Changpeng Liu wrote:
> For PCI prefetchable resources, Linux will create a
> write combined file as well, the library will try
> to map resourceX_wc file first, if the file does
> not exist, then it will map resourceX as usual.
> 
> Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
> ---
>  drivers/bus/pci/linux/pci_uio.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
>

Hi,

Given the lack of ordering guarantees with write-combined memory, I
would have thought that this is very risky to do without a complete set
of changes inside the PMDs to add in the necessary memory barriers to
ensure ordering of operations to the BARs.  Therefore, instead of
mapping one file or another, I think the change should be made to map
*both* in DPDK if available. Then each driver can chose whether to write
a given device register using uncacheable memory type or write-combining
memory type + any appropriate barriers.

For example, with many NICs the initialization of the device involves
many register writes in a pretty defined order, so wc operations are
probably to suitable as performance is not a concern. However, for data
path operations, a driver may chose to use wc memory for the occasional
device writes there, for performance reasons.

Regards,
/Bruce