Re: [RFC, PATCH 5/24] i386 Vmi code patching

From: Zachary Amsden
Date: Wed Mar 22 2006 - 23:01:48 EST

Chris Wright wrote:
* Zachary Amsden (zach@xxxxxxxxxx) wrote:
No, you don't need to dream up all the possible interface bits ahead of time. With a la carte interfaces, you can take what you need now, and add features later. You don't need an ABI for features. You need it for compatibility. You will need to update the hypervisor ABI. And you can't force people to upgrade their kernels.

How do you support an interface that's not already a part of the ABI
w/out changing the kernel?

You have to change the kernel for VMI interface upgrades - if you want to use the upgrades. You don't need to change the kernel for hypervisor ABI changes, nor does upgrading the interface require a kernel change. Interface upgrades are pretty easy to compartmentalize - you add block device support, you add a block device driver. Hypervisor ABI changes are not so easy, because of the data dependencies and potential for breaking compatibility. The massive security hole scenario is a good example of why you would need to break compatibility, but any number of things might make you want to change the hypervisor ABI.

The point of the VMI is to isolate the kernel from those changes, allowing kernel development to proceed unhindered, and allowing hypervisor innovation to thrive simultaneously.

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