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

From: Avi Kivity <avi@redhat.com>
To: Gregory Haskins <gregory.haskins@gmail.com>
Cc: Hollis Blanchard <hollisb@us.ibm.com>,
	Anthony Liguori <anthony@codemonkey.ws>,
	Gregory Haskins <ghaskins@novell.com>,
	Chris Wright <chrisw@sous-sol.org>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [RFC PATCH 0/3] generic hypercall support
Date: Mon, 11 May 2009 20:53:32 +0300	[thread overview]
Message-ID: <4A08661C.1000208@redhat.com> (raw)
In-Reply-To: <4A086065.2090600@gmail.com>

Gregory Haskins wrote:
> Avi Kivity wrote:
>   
>> Hollis Blanchard wrote:
>>     
>>> I haven't been following this conversation at all. With that in mind...
>>>
>>> AFAICS, a hypercall is clearly the higher-performing option, since you
>>> don't need the additional memory load (which could even cause a page
>>> fault in some circumstances) and instruction decode. That said, I'm
>>> willing to agree that this overhead is probably negligible compared to
>>> the IOp itself... Ahmdal's Law again.
>>>   
>>>       
>> 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)
>>
>>     
> Yeah, I agree with all this.  I am still wrestling with how to deal with
> the device-assignment problem w.r.t. shunting io requests into a
> hypercall vs letting them PF.  Are you saying we could simply ignore
> this case by disabling "MMIOoHC" when assignment is enabled?  That would
> certainly make the problem much easier to solve.
>   

No, we need to deal with hotplug.  Something like IO_COND that Chris 
mentioned, but how to avoid turning this into a doctoral thesis.

(On the other hand, device assignment requires the iommu, and I think 
you have to specify that up front?)

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