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

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.


Julien Grall

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