Re: [Xen-devel] [PATCH 02/16] SUPPORT.md: Add core functionality

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 11/21/2017 08:03 AM, Jan Beulich wrote:
>>>> On 13.11.17 at 16:41, <george.dunlap@xxxxxxxxxx> wrote:
>> --- a/SUPPORT.md
>> +++ b/SUPPORT.md
>> @@ -16,6 +16,65 @@ for the definitions of the support status levels etc.
>>  
>>  # Feature Support
>>  
>> +## Memory Management
>> +
>> +### Memory Ballooning
>> +
>> +    Status: Supported
> 
> Is this a proper feature in the context we're talking about? To me
> it's meaningful in guest OS context only. I also wouldn't really
> consider it "core", but placement within the series clearly is a minor
> aspect.
> 
> I'd prefer this to be dropped altogether as a feature, but

This doesn't make any sense to me.  Allowing a guest to modify its own
memory requires a *lot* of support, spread throughout the hypervisor;
and there are a huge number of recent security holes that would have
been much more difficult to exploit if guests didn't have the ability to
balloon up or down.

If what you mean is *specifically* the technique of making a "memory
balloon" to trick the guest OS into handing back memory without knowing
it, then it's just a matter of semantics.  We could call this "dynamic
memory control" or something like that if you prefer (although we'd have
to mention ballooning in the description to make sure people can find it).

> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> is independent of that.
> 
>> +### Credit2 Scheduler
>> +
>> +    Status: Supported
> 
> Sort of unrelated, but with this having been the case since 4.8 as it
> looks, is there a reason it still isn't the default scheduler?
Well first of all it was missing some features which credit1 had:
namely, soft affinity (i.e., required for host NUMA awareness) and caps.
 These were checked in this release cycle; but we also wanted to switch
the default at the beginning of a development cycle to get the highest
chance of shaking out any weird bugs.

So according to those criteria, we could switch to credit2 being the
default scheduler as soon as 4.10 development window opens.

At some point recently Dario said there were still some unusual behavior
he wanted to dig into; but I think with him not working for Citrix
anymore, it's doubtful we'll have resource to take that up; the best
option might be to just pull the lever and see what happens.

 -George

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