Re: [Xen-devel] Xen Project policy on feature flags

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.09.14 at 16:26, <david.vrabel@xxxxxxxxxx> wrote:
> On 26/09/14 14:24, Stefano Stabellini wrote:
>> I always thought that any XENFEAT feature flags could be removed going
>> forward, if the hypervisor maintainers decide to do so. However Ian
>> Campbell is of the opposite opinion, so I think we should have a clear
>> policy regarding them.
> 
> A guest that runs on Xen version V /must/ continue to run on V+1.
> 
> The is similar to the policy the Linux kernel has for the user space ABI.

Linux has proven a number of times that it doesn't always care about
such compatibility. The most prominent example being a major sysfs
layout rework in 2.6.37 which broke many older systems of mine.

I view feature flags much more like CPUs announcing certain
capabilities via CPUID - this is no guarantee that all that functionality
will also be present in future CPUs.

Jan


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