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

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Date:	Fri, 08 May 2009 14:02:04 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Avi Kivity <avi@...hat.com>
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

Avi Kivity wrote:
> 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.

Right, the basic math still stands.

>>
>> 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.

Yes, I misunderstood that they actually emulated it like that.  However, 
ia64 has no paravirtualization support today so surely, we aren't going 
to be justifying this via ia64, right?

Regards,

Anthony Liguori

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