On Wed, Sep 16, 2020 at 03:09:14PM +0100, Lorenzo Pieralisi wrote: > On Wed, Sep 16, 2020 at 09:12:26AM -0300, Jason Gunthorpe wrote: > > [...] > > > > You mean the driver uses a different path to the HW which ahs that > > > overhead, not that MMIOs have that overhead right ? > > > > The different path has overhead of doing extra useless MMIOs because > > they don't combine > > For my own information, this is IB user space driver code, correct ? Yes, maybe DPDK too > It tries to mmap buffer as WC and if it succeeds write into it in an > optimized fashion (that is just pure overhead on platforms where > normal NC memory - ie WC on arm64 - does not do what the > _architecture_ defines it should). Right, pure overhead if large PCI-E TLPs are not delivered to the device. Jason
