netdev - Re: [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for vbus

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Message-ID: <4A8A8631.1040006@redhat.com>
Date:	Tue, 18 Aug 2009 13:45:05 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Gregory Haskins <gregory.haskins@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	alacrityvm-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for	vbus_driver
 objects

On 08/18/2009 01:28 PM, Michael S. Tsirkin wrote:
>
>> Suppose a nested guest has two devices.  One a virtual device backed by
>> its host (our guest), and one a virtual device backed by us (the real
>> host), and assigned by the guest to the nested guest.  If both devices
>> use hypercalls, there is no way to distinguish between them.
>>      
> Not sure I understand. What I had in mind is that devices would have to
> either use different hypercalls and map hypercall to address during
> setup, or pass address with each hypercall.  We get the hypercall,
> translate the address as if it was pio access, and know the destination?
>    

There are no different hypercalls.  There's just one hypercall 
instruction, and there's no standard on how it's used.  If a nested call 
issues a hypercall instruction, you have no idea if it's calling a 
Hyper-V hypercall or a vbus/virtio kick.

You could have a protocol where you register the hypercall instruction's 
address with its recipient, but it quickly becomes a tangled mess.  And 
for what?  pio and hypercalls have the same performance characteristics.

-- 
error compiling committee.c: too many arguments to function

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