On 02/01/2018 12:59 PM, Bruce Richardson wrote:
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....@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.
+1
I think so too that it would be useful to have both mappings available and
allow driver to choose which one to use.
