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

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Date:	Mon, 11 May 2009 21:02:29 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Hollis Blanchard <hollisb@...ibm.com>,
	Gregory Haskins <gregory.haskins@...il.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Chris Wright <chrisw@...s-sol.org>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] generic hypercall support

Anthony Liguori wrote: 
>>
>> It's a question of cost vs. benefit.  It's clear the benefit is low 
>> (but that doesn't mean it's not worth having).  The cost initially 
>> appeared to be very low, until the nested virtualization wrench was 
>> thrown into the works.  Not that nested virtualization is a reality 
>> -- even on svm where it is implemented it is not yet production 
>> quality and is disabled by default.
>>
>> Now nested virtualization is beginning to look interesting, with 
>> Windows 7's XP mode requiring virtualization extensions.  Desktop 
>> virtualization is also something likely to use device assignment 
>> (though you probably won't assign a virtio device to the XP instance 
>> inside Windows 7).
>>
>> Maybe we should revisit the mmio hypercall idea again, it might be 
>> workable if we find a way to let the guest know if it should use the 
>> hypercall or not for a given memory range.
>>
>> mmio hypercall is nice because
>> - it falls back nicely to pure mmio
>> - it optimizes an existing slow path, not just new device models
>> - it has preexisting semantics, so we have less ABI to screw up
>> - for nested virtualization + device assignment, we can drop it and 
>> get a nice speed win (or rather, less speed loss)
>
> If it's a PCI device, then we can also have an interrupt which we 
> currently lack with vmcall-based hypercalls.  This would give us 
> guestcalls, upcalls, or whatever we've previously decided to call them.

Sorry, I totally failed to understand this.  Please explain.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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