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

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Message-ID: <4A047EDE.2020806@redhat.com>
Date:	Fri, 08 May 2009 21:50:06 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Gregory Haskins <ghaskins@...ell.com>,
	Marcelo Tosatti <mtosatti@...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

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.

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