Re: [Xen-devel] (v2) Design proposal for RMRR fix

[George Dunlap <dunlapg@xxxxxxxxx>]

On Mon, Jan 12, 2015 at 11:22 AM, Tian, Kevin <kevin.tian@xxxxxxxxx> wrote:
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Monday, January 12, 2015 6:23 PM
>>
>> >>> On 12.01.15 at 11:12, <kevin.tian@xxxxxxxxx> wrote:
>> >>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> Sent: Monday, January 12, 2015 6:09 PM
>> >>
>> >> >>> On 12.01.15 at 10:56, <kevin.tian@xxxxxxxxx> wrote:
>> >> > the result is related to another open whether we want to block guest
>> >> > boot for such problem. If 'warn' in domain builder is acceptable, we
>> >> > don't need to change lowmem for such rare 1GB case, just throws
>> >> > a warning for unnecessary conflictions (doesn't hurt if user doesn't
>> >> > assign it).
>> >>
>> >> And how would you then deal with the one guest needing that
>> >> range reserved?
>> >
>> > if guest needs the range, then report-all or report-sel doesn't matter.
>> > domain builder throws the warning, and later device assignment will
>> > fail (or warn w/ override). In reality I think 1GB is rare. Making such
>> > assumption to simplify implementation is reasonable.
>>
>> One of my main problems with all you recent argumentation here
>> is the arbitrary use of the 1Gb boundary - there's nothing special
>> in this discussion with where the boundary is. Everything revolves
>> around the (undue) effect of report-all on domains not needing all
>> of the ranges found on the host.
>>
>
> I'm not sure which part of my argument is not clear here. report-all
> would be a problem here only if we want to fix all the conflictions
> (then pulling unnecessary devices increases the confliction possibility)
> in the domain builder. but if we only fix reasonable ones (e.g. >3GB)
> while warn other conflictions (e.g. <3G) in domain builder (let later
> assignment path to actually fail if confliction does matter), then we
> don't need to solve all conflictions in domain builder (if say 1G example
> fixing it may instead reduce lowmem greatly) and then report-all
> may just add more warnings than report-sel for unused devices.

You keep saying "report-all" or "report-sel", but I'm not 100% clear
what you mean by those.  In any case, the naming has got to be a bit
misleading: the important questions at the moment, AFAICT, are:

1. Whether we make holes at boot time for all RMRRs on the system, or
whether only make RMRRs for some subset (or potentially some other
arbitrary range, which may include RMRRs on other hosts to which we
may want to migrate).

2. Whether those holes are made by the domain builder in libxc, or by hvmloader

3. What happens if Xen is asked to assign a device and it finds that
the required RMRR is not empty:
 a. during guest creation
 b. after the guest has booted

Obviously at some point some part of the toolstack needs to identify
which RMRRs go with what device, so that either libxc or hvmloader can
make the appropriate holes in the address space; but at that point,
"report" is not so much the right word as "query".  (Obviously we want
to "report" in the e820 map all RMRRs that we've made holes for in the
guest.)

 -George

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