On Wed, Sep 27, 2017 at 09:43:44AM +0000, David Laight wrote: > From: Segher Boessenkool > > Sent: 27 September 2017 10:28 > ... > > You also need nasty code to deal with the start and end of strings, with > > conditional branches and whatnot, which quickly overwhelms the benefit > > of using vector registers at all. This tradeoff also changes with newer > > ISA versions. > > The goal posts keep moving. > For instance with modern intel x86 cpus 'rep movsb' is by far the fastest > way to copy data (from cached memory). > > > Things have to become *really* cheap before it will be good to often use > > vector registers in the kernel though. > > I've had thoughts about this in the past. > If the vector registers belong to the current process then you might > get away with just saving the ones you want to use. > If they belong to a different process then you also need to tell the > FPU save code where you've saved the registers. > Then the IPI code can recover all the correct values. > > On X86 all the AVX registers are caller saved, the system call > entry could issue the instruction that invalidates them all. > Kernel code running in the context of a user process could then > use the registers without saving them. > It would only need to set a mark to ensure they are invalidated > again on return to user (might be cheap enough to do anyway). > Dunno about PPC though.
I am not aware of any ppc instruction which can set a "mark" or provide any high granularity flag against single or subgroup of vec regs' validity. But ppc experts may want to correct me. Thanks, - Simon
