linux-kernel - Re: [RFC PATCH 0/3] generic hypercall support

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Date:	Mon, 11 May 2009 11:35:12 -0500
From:	Hollis Blanchard <hollisb@...ibm.com>
To:	Gregory Haskins <gregory.haskins@...il.com>
Cc:	Anthony Liguori <anthony@...emonkey.ws>,
	Gregory Haskins <ghaskins@...ell.com>,
	Avi Kivity <avi@...hat.com>,
	Chris Wright <chrisw@...s-sol.org>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] generic hypercall support

On Mon, 2009-05-11 at 09:14 -0400, Gregory Haskins wrote:
> 
> >> for request-response, this is generally for *every* packet since
> you
> >> cannot exploit buffering/deferring.
> >>
> >> Can you back up your claim that PPC has no difference in
> performance
> >> with an MMIO exit and a "hypercall" (yes, I understand PPC has no
> "VT"
> >> like instructions, but clearly there are ways to cause a trap, so
> >> presumably we can measure the difference between a PF exit and
> something
> >> more explicit).
> >>   
> >
> > First, the PPC that KVM supports performs very poorly relatively
> > speaking because it receives no hardware assistance
> 
> So wouldn't that be making the case that it could use as much help as
> possible?

I think he's referencing Ahmdal's Law here. While I'd agree, this is
relevant only for the current KVM PowerPC implementations. I think it
would be short-sighted to design an IO architecture around that.

> >   this is not the right place to focus wrt optimizations.
> 
> Odd choice of words.  I am advocating the opposite (broad solution to
> many arches and many platforms (i.e. hypervisors)) and therefore I am
> not "focused" on it (or really any one arch) at all per se.  I am
> _worried_ however, that we could be overlooking PPC (as an example) if
> we ignore the disparity between MMIO and HC since other higher
> performance options are not available like PIO.  The goal on this
> particular thread is to come up with an IO interface that works
> reasonably well across as many hypervisors as possible.  MMIO/PIO do
> not appear to fit that bill (at least not without tunneling them over
> HCs)

I haven't been following this conversation at all. With that in mind...

AFAICS, a hypercall is clearly the higher-performing option, since you
don't need the additional memory load (which could even cause a page
fault in some circumstances) and instruction decode. That said, I'm
willing to agree that this overhead is probably negligible compared to
the IOp itself... Ahmdal's Law again.

-- 
Hollis Blanchard
IBM Linux Technology Center

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