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Date: Fri, 08 May 2009 21:50:06 +0300
From: Avi Kivity <avi@redhat.com>
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To: Anthony Liguori <anthony@codemonkey.ws>
CC: Gregory Haskins <ghaskins@novell.com>,
       Marcelo Tosatti <mtosatti@redhat.com>,
       Chris Wright <chrisw@sous-sol.org>,
       Gregory Haskins <gregory.haskins@gmail.com>,
       linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [RFC PATCH 0/3] generic hypercall support
References: <4A0040C0.1080102@redhat.com> <4A0041BA.6060106@novell.com> <4A004676.4050604@redhat.com> <4A0049CD.3080003@gmail.com> <20090505231718.GT3036@sequoia.sous-sol.org> <4A010927.6020207@novell.com> <20090506072212.GV3036@sequoia.sous-sol.org> <4A018DF2.6010301@novell.com> <20090506160712.GW3036@sequoia.sous-sol.org> <4A031471.7000406@novell.com> <20090507233503.GA9103@amt.cnet> <4A043E89.90403@novell.com> <4A044786.2080508@codemonkey.ws>
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Anthony Liguori wrote:
>
> And we're now getting close to the point where the difference is 
> virtually meaningless.
>
> At .14us, in order to see 1% CPU overhead added from PIO vs HC, you 
> need 71429 exits.
>

If I read things correctly, you want the difference between PIO and 
PIOoHC, which is 210ns.  But your point stands, 50,000 exits/sec will 
add 1% cpu overhead.

>
> The non-x86 architecture argument isn't valid because other 
> architectures either 1) don't use PCI at all (s390) and are already 
> using hypercalls 2) use PCI, but do not have a dedicated hypercall 
> instruction (PPC emb) or 3) have PIO (ia64).

ia64 uses mmio to emulate pio, so the cost may be different.  I agree on 
x86 it's almost negligible.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.