Re: [Qemu-devel] [RFC] Next gen kvm api

[Alan Cox]

> >    register_pio_hook_ptr_r(PIO_IDE, SIZE_BYTE,&s->cmd[0]);
> >    for (i = 1; i<  7; i++) {
> >      register_pio_hook_ptr_r(PIO_IDE + i, SIZE_BYTE,&s->cmd[i]);
> >      register_pio_hook_ptr_w(PIO_IDE + i, SIZE_BYTE,&s->cmd[i]);
> >    }
> 
> You can't easily serialize updates to that address with the kernel since two 
> threads are likely going to be accessing it at the same time.  That either 
> means 
> an expensive sync operation or a reliance on atomic instructions.

Who cares

If your API is right this isn't a problem (and for IDE the guess that it
won't happen you will win 99.999% of the time).

In fact IDE you can do even better in many cases because you'll get a
single rep outsw you can trap and shortcut.

> But not all architectures offer non-word sized atomic instructions so it gets 
> fairly nasty in practice.

Thats their problem. We don't screwup the fast paths because some
hardware vendor screwed up that bit of their implementation. That's
*their* problem not everyone elses.

So on x86 IDE should be about 10 outb traps that can be predicted, a rep
outsw which can be shortcut and a completion set of inb/inw ops that can
be predicted.

You should hit userspace about once per IDE operation. Fix the hot paths
with good design and the noise doesn't matter.

Alan