On Tue, 2015-10-06 at 22:30 +0200, christophe leroy wrote: > Le 06/10/2015 18:46, Scott Wood a écrit : > > On Tue, 2015-10-06 at 15:35 +0200, Christophe Leroy wrote: > > > Le 29/09/2015 00:07, Scott Wood a écrit : > > > > On Tue, Sep 22, 2015 at 06:50:29PM +0200, Christophe Leroy wrote: > > > > > We are spending between 40 and 160 cycles with a mean of 65 cycles > > > > > in > > > > > the TLB handling routines (measured with mftbl) so make it more > > > > > simple althought it adds one instruction. > > > > > > > > > > Signed-off-by: Christophe Leroy <christophe.le...@c-s.fr> > > > > Does this just make it simpler or does it make it faster? What is the > > > > performance impact? Is the performance impact seen with or without > > > > CONFIG_8xx_CPU6 enabled? Without it, it looks like you're adding an > > > > mtspr/mfspr combo in order to replace one mfspr. > > > > > > > > > > > The performance impact is not noticeable. Theoritically it adds 1 cycle > > > on a mean of 65 cycles, that is 1.5%. Even in the worst case where we > > > spend around 10% of the time in TLB handling exceptions, that represents > > > only 0.15% of the total CPU time. So that's almost nothing. > > > Behind the fact to get in simpler, the main reason is because I need a > > > third register for the following patch in the set, otherwise I would > > > spend a more time saving and restoring CR several times. > > FWIW, the added instruction is an SPR access and I doubt that's only one > > cycle. > > > > > According to the mpc885 reference manual (table 9-1), Instruction > Execution Timing for "Move to: mtspr, mtcrf, mtmsr, mcrxr except mtspr to LR > and CTR and to SPRs external to the core" is "serialize + 1 cycle". > Taking into account we preeceeding instructions are also 'mtspr', we are > already serialized, so it is only one cycle I believe. > Am I interpreting it wrong ?
I don't know. The manual doesn't go into much detail about the mechanics of serialization. If it's just about "block[ing] all execution units" without any effect on fetching, decoding, etc. then maybe you're right. -Scott _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
