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

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Date:	Wed, 8 Feb 2012 17:20:37 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Alexander Graf <agraf@...e.de>, Avi Kivity <avi@...hat.com>,
	qemu-devel <qemu-devel@...gnu.org>,
	kvm-ppc <kvm-ppc@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api

> >    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
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