linux-kernel - Re: [PATCHv5 3/3] vhost_net: a kernel-level virtio server

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Date:	Mon, 14 Sep 2009 12:08:55 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"Ira W. Snyder" <iws@...o.caltech.edu>, netdev@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu, linux-mm@...ck.org,
	akpm@...ux-foundation.org, hpa@...or.com,
	Rusty Russell <rusty@...tcorp.com.au>, s.hetze@...ux-ag.com
Subject: Re: [PATCHv5 3/3] vhost_net: a kernel-level virtio server

Michael S. Tsirkin wrote:
> On Fri, Sep 11, 2009 at 12:00:21PM -0400, Gregory Haskins wrote:
>> FWIW: VBUS handles this situation via the "memctx" abstraction.  IOW,
>> the memory is not assumed to be a userspace address.  Rather, it is a
>> memctx-specific address, which can be userspace, or any other type
>> (including hardware, dma-engine, etc).  As long as the memctx knows how
>> to translate it, it will work.
> 
> How would permissions be handled?

Same as anything else, really.  Read on for details.

> it's easy to allow an app to pass in virtual addresses in its own address space.

Agreed, and this is what I do.

The guest always passes its own physical addresses (using things like
__pa() in linux).  This address passed is memctx specific, but generally
would fall into the category of "virtual-addresses" from the hosts
perspective.

For a KVM/AlacrityVM guest example, the addresses are GPAs, accessed
internally to the context via a gfn_to_hva conversion (you can see this
occuring in the citation links I sent)

For Ira's example, the addresses would represent a physical address on
the PCI boards, and would follow any kind of relevant rules for
converting a "GPA" to a host accessible address (even if indirectly, via
a dma controller).


>  But we can't let the guest specify physical addresses.

Agreed.  Neither your proposal nor mine operate this way afaict.

HTH

Kind Regards,
-Greg


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