On Thu, Apr 14, 2016 at 11:15:21AM +0100, Bruce Richardson wrote: > An analysis of the i40e code using Intel? VTune? Amplifier 2016 showed > that the code was unexpectedly causing stalls due to "Loads blocked by > Store Forwards". This can occur when a load from memory has to wait > due to the prior store being to the same address, but being of a smaller > size i.e. the stored value cannot be directly returned to the loader. > [See ref: https://software.intel.com/en-us/node/544454] > > These stalls are due to the way in which the data_len values are handled > in the driver. The lengths are extracted using vector operations, but those > 16-bit lengths are then assigned using scalar operations i.e. 16-bit > stores. > > These regular 16-bit stores actually have two effects in the code: > * they cause the "Loads blocked by Store Forwards" issues reported > * they also cause the previous loads in the RX function to actually be a > load followed by a store to an address on the stack, because the 16-bit > assignment can't be done to an xmm register. > > By converting the 16-bit stores operations into a sequence of SSE blend > operations, we can ensure that the descriptor loads only occur once, and > avoid both the additional store and loads from the stack, as well as the > stalls due to the second loads being blocked. > > Signed-off-by: Bruce Richardson <bruce.richardson at intel.com> > Self-NAK on this version. The blend instruction used is SSE4.1 so breaks the "default" build.
Two obvious options to fix this: 1. Keep the old code with SSE4.1 #ifdefs separating old and new 2. Update the vpmd requirement to SSE4.1, and factor that in during runtime select of the RX code path. Personally, I prefer the second option. Any objections? /Bruce