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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: George Dunlap
> Sent: Monday, January 12, 2015 8:14 PM
> 
> 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:

I explained them in original proposal

> 
> 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).

I use 'report-all' to stand for making holes for all RMRRs on the system,
while 'report-sel' for specified subset.

including other RMRRs (from admin for migration) is orthogonal to
above open.

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

based on current discussion, whether to make holes in hvmloader
doesn't bring fundamental difference. as long as domain builder
still need to populate memory (even minimal for hvmloader to boot),
it needs to check conflict and may ideally make hole too (though we
may make assumption not doing that)

> 
> 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

for Xen we don't need differentiate a/b. by default it's clear failure
should be returned as it implies a security/correctness issue if
moving forward. but based on discussion an override to 'warn' only
is preferred, so admin can make decision (remains an open on
whether to do global override or per-device override)

> 
> 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.)

yes, using 'report' doesn't catch all the changes we need to make. Just
use them to simplify discussion in case all are on the same page. However
clearly my original explanation didn't make it. :/

and state my major intention again. I don't think the preparation (i.e.
detect confliction and make holes) for device assignment should be a 
a blocking failure. Throw warning should be enough (i.e. in libxc). We
should let actual device assignment path to make final call based on
admin's configuration (default 'fail' w/ 'warn' override). Based on that
policy I think 'report-all' (making holes for all host RMRRs) is an
acceptable approach, w/ small impact on possibly more warning 
messages (actually not bad to help admin understand the hotplug
possibility on this platform) and show more reserved regions to the
end user (but he shouldn't make any assumption here). :-)

Thanks
Kevin
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