Re: [Xen-devel] Alternate p2m design specification

[Ed White <edmund.h.white@xxxxxxxxx>]

On 06/11/2015 12:05 AM, Jan Beulich wrote:
>>>> On 10.06.15 at 18:39, <edmund.h.white@xxxxxxxxx> wrote:
>> On 06/10/2015 12:43 AM, Jan Beulich wrote:
>>>>>> On 10.06.15 at 02:09, <edmund.h.white@xxxxxxxxx> wrote:
>>>> Design
>>>> ======
>>>
>>> Reads all quite reasonable; just one minor remark:
>>>
>>>> - Core altp2m functionality
>>>>
>>>> A new altp2m type is added to the p2m types (in addition to the previous 
>>>> hostp2m and nestedp2m types). An HVM domain can be started in hostp2m mode 
>>>> and switched over into altp2m mode via a hypercall. Once a HVM domain is 
>>>> in 
>>>> altp2m mode, a set of (currently set size is 10) altp2m objects is managed 
>> by 
>>>> Xen.
>>>
>>> Rather than hardcoding 10, how about making the Xen command line
>>> option specify the value (instead of being a simple boolean one)?
>>
>> For the use cases we're currently aware of, even 10 is generous. I'd be
>> wary of allowing the user to specify any number up to and including 512
>> with the current implementation, because there are a number of places where
>> we do a linear search of the list.
> 
> I was actually considering this to allow the user to reduce the value
> below 10.
> 

Understood. I chose 10 as the initial value because it's high enough for
all the likely use cases we know of, and low enough not be too expensive
at run time. My concern with allowing the value to be set lower is that
it would cause users of the code to fail in unexpected ways.

>> In practice, 1 alternate p2m is not useful, so in a future version of the
>> code, when we have more idea of how useful people find it, we could say
>> 0=off, 1=on with 10 alternates, any other number=on with that many
>> alternates.
>>
>> Does that seem reasonable?
> 
> That's certainly an option, but using only a number from the beginning
> would simplify the handling of the option in the hypervisor. But of
> course that's only a very minor aspect.
> 

The reason I would like to avoid doing it now is simply development and
test time. I'm trying to avoid dropping a large amount of code on you
and the other maintainers and/or reviewers right at the function freeze
deadline, and it's already challenging to meet that aim with the amount
of work still left to do.

Ed


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