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:

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On Thu, Jan 25, 2018 at 08:54:13PM +0100, Christoffer Dall wrote:

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On Tue, Jan 23, 2018 at 04:04:40PM +0000, Dave Martin wrote:

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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.

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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".)

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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