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

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Date:	Thu, 07 May 2009 14:37:40 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Gregory Haskins <gregory.haskins@...il.com>,
	Chris Wright <chrisw@...s-sol.org>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [RFC PATCH 0/3] generic hypercall support

Avi Kivity wrote:
> Gregory Haskins wrote:
>> I guess technically mmio can just be a simple access of the page which
>> would be problematic to trap locally without a PF.  However it seems
>> that most mmio always passes through a ioread()/iowrite() call so this
>> is perhaps the hook point.  If we set the stake in the ground that mmios
>> that go through some other mechanism like PFs can just hit the "slow
>> path" are an acceptable casualty, I think we can make that work.
>>   
>
> That's my thinking exactly.

Cool,  I will code this up and submit it.  While Im at it, Ill run it
through the "nullio" ringer, too. ;)  It would be cool to see the
pv-mmio hit that 2.07us number.  I can't think of any reason why this
will not be the case.

>
> Note we can cheat further.  kvm already has a "coalesced mmio" feature
> where side-effect-free mmios are collected in the kernel and passed to
> userspace only when some other significant event happens.  We could
> pass those addresses to the guest and let it queue those writes
> itself, avoiding the hypercall completely.
>
> Though it's probably pointless: if the guest is paravirtualized enough
> to have the mmio hypercall, then it shouldn't be using e1000.

Yeah...plus at least for my vbus purposes, all my my guest->host
transitions are explicitly to cause side-effects, or I wouldn't be doing
them in the first place ;)  I suspect virtio-pci is exactly the same. 
I.e. the coalescing has already been done at a higher layer for
platforms running "PV" code.

Still a cool feature, tho.

-Greg



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