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

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Date:	Mon, 11 May 2009 11:44:20 -0500
From:	Hollis Blanchard <hollisb@...ibm.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Gregory Haskins <ghaskins@...ell.com>, Avi Kivity <avi@...hat.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Gregory Haskins <gregory.haskins@...il.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] generic hypercall support

On Sun, 2009-05-10 at 13:38 -0500, Anthony Liguori wrote:
> Gregory Haskins wrote:
> >
> > 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  this is not the 
> right place to focus wrt optimizations.
> 
> And because there's no hardware assistance, there simply isn't a 
> hypercall instruction.  Are PFs the fastest type of exits?  Probably not 
> but I honestly have no idea.  I'm sure Hollis does though.

Memory load from the guest context (for instruction decoding) is a
*very* poorly performing path on most PowerPC, even considering server
PowerPC with hardware virtualization support. No, I don't have any data
for you, but switching the hardware MMU contexts requires some
heavyweight synchronization instructions.

> Page faults are going to have tremendously different performance 
> characteristics on PPC too because it's a software managed TLB. There's 
> no page table lookup like there is on x86.

To clarify, software-managed TLBs are only found in embedded PowerPC.
Server and classic PowerPC use hash tables, which are a third MMU type.

-- 
Hollis Blanchard
IBM Linux Technology Center

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