Re: [Xen-devel] [PATCH, v2] add privileged/unprivileged kernel feature indication

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Thu, 2011-07-21 at 09:55 +0100, Jan Beulich wrote:
> >>> On 21.07.11 at 10:38, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> > On Thu, 2011-07-21 at 09:16 +0100, Jan Beulich wrote:
> >> > You also have not explained _why_ a dom0-only guest would be a useful
> >> > thing to have and to add extra complexity to our interfaces for, it's
> >> > obviously very much a corner case.
> >> 
> >> It's really a decision between having efficient code (i.e. as little
> >> unused code as possible in a kernel suiting a particular need) and
> >> having a (relatively) general-purpose kernel.
> > 
> > We are talking about half a dozen lines of code to spit out a static
> > string ("I don't support domU operation") to the domU and/or a guest
> 
> You still didn't tell where such a message would show up: Printing
> such a message isn't a big deal, but pointless if it goes into no-where.
> If instead the hypervisor (for Dom0) or the tools (for DomU-s) print
> something, this will be visible in a known place.

If someone runs a dom0-only kernel as a domU (presumably by mistake)
then they are naturally going to look in the domU console ring for error
messages when it doesn't work (because they thought they were running a
domU, where else would they look?). Placing that message there is
basically a memcpy and an evtchn_notify.

If someone runs a domU only kernel as a dom0 then the XENFEAT_privileged
(or whatever it gets called) is precisely enough to allow the hypervisor
to say something useful in that case.

Ian.



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