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

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Message-ID: <4A086C07.7050103@codemonkey.ws>
Date:	Mon, 11 May 2009 13:18:47 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Avi Kivity <avi@...hat.com>
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

Avi Kivity wrote:
> 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.

I totally missed what you meant by MMIO hypercall.

In what cases do you think MMIO hypercall would result in a net benefit?

I think the difference in MMIO vs hcall will be overshadowed by the 
heavy weight transition to userspace.  The only thing I can think of 
where it may matter is for in-kernel devices like the APIC but that's a 
totally different path in Linux.

Regards,

Anthony Liguori
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