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

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Date:	Tue, 05 May 2009 18:02:14 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Gregory Haskins <gregory.haskins@...il.com>
CC:	Gregory Haskins <ghaskins@...ell.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] generic hypercall support

Gregory Haskins wrote:
> I see.  I had designed it slightly different where KVM could assign any
> top level vector it wanted and thus that drove the guest-side interface
> you see here to be more "generic hypercall".  However, I think your
> proposal is perfectly fine too and it makes sense to more narrowly focus
> these calls as specifically "dynamic"...as thats the only vectors that
> we could technically use like this anyway.
>
> So rather than allocate a top-level vector, I will add "KVM_HC_DYNAMIC"
> to kvm_para.h, and I will change the interface to follow suit (something
> like s/hypercall/dynhc).  Sound good?
>   

Yeah.

Another couple of points:

- on the host side, we'd rig this to hit an eventfd.  Nothing stops us 
from rigging pio to hit an eventfd as well, giving us kernel handling 
for pio trigger points.
- pio actually has an advantage over hypercalls with nested guests.  
Since hypercalls don't have an associated port number, the lowermost 
hypervisor must interpret a hypercall as going to a guest's hypervisor, 
and not any lower-level hypervisors.  What it boils down to is that you 
cannot use device assignment to give a guest access to a virtio/vbus 
device from a lower level hypervisor.

(Bah, that's totally unreadable.  What I want is

instead of

   hypervisor[eth0/virtio-server]   ----> 
intermediate[virtio-driver/virtio-server]  ----> guest[virtio-driver]

do

   hypervisor[eth0/virtio-server]   ----> intermediate[assign virtio 
device]  ----> guest[virtio-driver]

well, it's probably still unreadable)

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

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