linux-kernel - Re: [PATCH 03/16] KVM-HDR: KVM Userspace registering ioctl

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Date:	Wed, 26 Jan 2011 13:23:26 -0200
From:	Glauber Costa <glommer@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	aliguori@...ibm.com
Subject: Re: [PATCH 03/16] KVM-HDR: KVM Userspace registering ioctl

On Wed, 2011-01-26 at 17:14 +0200, Avi Kivity wrote:
> On 01/26/2011 02:14 PM, Glauber Costa wrote:
> > On Wed, 2011-01-26 at 13:12 +0200, Avi Kivity wrote:
> > >  On 01/24/2011 08:06 PM, Glauber Costa wrote:
> > >  >  KVM, which stands for KVM Virtual Memory (I wanted to call it KVM Virtual Mojito),
> > >  >  is a piece of shared memory that is visible to both the hypervisor and the guest
> > >  >  kernel - but not the guest userspace.
> > >  >
> > >  >  The basic idea is that the guest can tell the hypervisor about a specific
> > >  >  piece of memory, and what it expects to find in there. This is a generic
> > >  >  abstraction, that goes to userspace (qemu) if KVM (the hypervisor) can't
> > >  >  handle a specific request, thus giving us flexibility in some features
> > >  >  in the future.
> > >  >
> > >  >  KVM (The hypervisor) can change the contents of this piece of memory at
> > >  >  will. This works well with paravirtual information, and hopefully
> > >  >  normal guest memory - like last update time for the watchdog, for
> > >  >  instance.
> > >  >
> > >  >  This patch contains the header part of the userspace communication implementation.
> > >  >  Userspace can query the presence/absence of this feature in the normal way.
> > >  >  It also tells the hypervisor that it is capable of handling - in whatever
> > >  >  way it chooses, registrations that the hypervisor does not know how to.
> > >  >  In x86, only user so far, this mechanism is implemented as generic userspace
> > >  >  msr exit, that could theorectically be used to implement msr-handling in
> > >  >  userspace.
> > >  >
> > >  >  I am keeping the headers separate to facilitate backports to people
> > >  >  who wants to backport the kernel part but not the hypervisor, or the other way around.
> > >  >
> > >
> > >  Again the protocol is not specified.  How about starting from
> > >  Documentation/kvm/api.txt so we don't have to guess?
> > I will do that in the next version, if the idea is not shoot up
> > completely.
> 
> For some reason people write the code first and the documentation as an 
> afterthought.  If you switch the order, you can get a high level review 
> first, followed by a low-level code review once the high level details 
> are agreed.  Of course it's hard to do major changes this way, since the 
> API often evolves while writing the code.

Thing is, processor runs code, not english. So I guess people write code
first at least to know what works vs what not works before writing a
potentially beautiful-but-only-theoretical novel.

Rest assured, I don't plan to push for merges of any feature before
providing docs.


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