Re: [Xen-devel] [PATCH 0/8] xen: don't let keyhandlers block indefinitely on locks

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 14/02/2020 09:37, Jan Beulich wrote:
> On 13.02.2020 19:38, Andrew Cooper wrote:
> > On 13/02/2020 12:54, Juergen Gross wrote:
> > > Keyhandlers dumping hypervisor information to the console often need
> > > to take locks while accessing data. In order to not block in case of
> > > system inconsistencies it is convenient to use trylock variants when
> > > obtaining the locks. On the other hand a busy system might easily
> > > encounter held locks, so this patch series is adding special trylock
> > > variants with a timeout used by keyhandlers.
> > This is a backwards step.
> >
> > Keyhandlers are for debugging purposes.  When debugging it is far more
> > important to get the requested data, than almost anything else.
> >
> > The system will cope with a multi-second outage occurring approximately
> > never.  A person debugging who can't get the data has no chance of
> > fixing whatever problem they are looking for.
> >
> > This series seems to be breaking the one critical usecase for
> > keyhandlers, to fix what - not let debugging get in the way of the
> > smooth running of the system?  A system in need of debugging in the
> > first place has bigger problems than needing to run smoothly.
> I certainly accept what you say further up, but I don't think this
> last statement is universally true. There may be a single guest in
> trouble, which - to find out about its state - some debugging keys
> may want issuing. Disturbing the host and all other guests for this
> is not a good idea imo.
This seems to suggest that you only want information for a single guest. 
Therefore using debugging keys was already a bad idea because it will 
disturb all the other guests.
For your setup, it might be worth considering to extend xenctx or 
introduce a way to dump information for a specific domain.
Cheers,

--
Julien Grall

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