On Tue, Feb 13, 2018 at 09:55:02AM +0100, Christoffer Dall wrote: > On Fri, Feb 09, 2018 at 04:17:39PM +0000, Dave Martin wrote: > > On Thu, Jan 25, 2018 at 08:54:13PM +0100, Christoffer Dall wrote: > > > On Tue, Jan 23, 2018 at 04:04:40PM +0000, Dave Martin wrote: [...] > > > > The individual accessor functions also become unnecessary in this case, > > > > because we wouldn't need to derive function pointers from them any > > > > more. > > > > > > > > I don't know how performance would compare in practice though. > > > > > > I don't know either. But I will say that the whole idea behind put/load > > > is that you do this rarely, and going to userspace from KVM is > > > notriously expensive, also on x86. > > > > I guess that makes sense. I'm still a bit hazy on the big picture > > for KVM. > > > > > > I'm also assuming that all calls to these accessors are const-foldable. > > > > If not, relying on inlining would bloat the generated code a lot. > > > > > > We have places where this is not the cae, access_vm_reg() for example. > > > But if we really, really, wanted to, we could rewrite that to have a > > > function for each register, but that's pretty horrid on its own. > > > > That might not be too bad if there is only one giant inline expansion > > and the rest are folded down. > > > > > > I guess this is something to revisit _if_ we suspect a performance > > bottleneck later on. > > > > For now, I was lacking some understanding regarding how this code gets > > run, so I was guessing about potential issues rather then proven > > issues. > > > > This was a very useful discussion. I think I'll change this to a big > switch statement in the header file using a static inline, because it > makes the code more readable, and if we notice a huge code size > explosion, we can take measures to make sure things are const-foldable. Sure, that sounds reasonable. C99 inline semantics allow a single out-of-line body to be linked in somewhere for when the function isn't inlined, so we might be able to mitigate the bloat that way if it's a problem... unless the compiler flags sabotage it (I remember GCC traditionally does something a bit different where there's a particular difference between "inline" and "extern inline".) > > As you might guess, I'm still at the "stupid questions" stage for > > this series :) > > Not at all. Hmmm, I must try to be more stupid when I look at the other patches... Cheers ---Dave
