Re: [PATCH] docs: specify numerical values of Xenstore commands

[Jürgen Groß]

On 06.03.25 14:09, Jan Beulich wrote:

On 06.03.2025 13:56, Jürgen Groß wrote:

On 06.03.25 13:32, Jan Beulich wrote:

On 06.03.2025 13:23, Juergen Gross wrote:

--- a/docs/misc/xenstore.txt
+++ b/docs/misc/xenstore.txt
@@ -86,6 +86,65 @@ parts of xenstore inaccessible to some clients.  In any case 
passing
   bulk data through xenstore is not recommended as the performance
   properties are poor.

+
+Below is a table with all defined Xenstore message types (type name
+and its associated numerical value).
+
+Some types are optional to be supported by a specific Xenstore
+implementation.  If an optional type is not supported by a Xenstore
+implementation, Xen tools will continue to work, maybe with slightly
+reduced functionality.  A mandatory type not being supported will
+result in severely reduced functionality, like inability to create
+domains.  In case a type is optional, this is stated in the table with
+the lost functionality in case Xenstore doesn't support that type.
+
+XS_CONTROL             0       optional
+       If not supported, xenstore-control command will not work.
+       XS_DEBUG is a deprecated alias of XS_CONTROL.
+XS_DIRECTORY           1
+XS_READ                        2
+XS_GET_PERMS           3
+XS_WATCH               4
+XS_UNWATCH             5
+XS_TRANSACTION_START   6
+XS_TRANSACTION_END     7
+XS_INTRODUCE           8
+XS_RELEASE             9
+XS_GET_DOMAIN_PATH     10
+XS_WRITE               11
+XS_MKDIR               12
+XS_RM                  13
+XS_SET_PERMS           14
+XS_WATCH_EVENT         15
+       Not valid in client sent messages.
+       Only valid in Xenstore replies.
+XS_ERROR               16
+       Not valid in client sent messages.
+       Only valid in Xenstore replies.
+XS_IS_DOMAIN_INTRODUCED        17
+XS_RESUME              18
+XS_SET_TARGET          19
+XS_RESTRICT            20      no longer supported
+       XS_RESTRICT has been removed, the type value 20 is invalid.
+XS_RESET_WATCHES       21
+XS_DIRECTORY_PART      22      optional
+       If not supported, the output of xenstore-ls might be incomplete
+       with moŕe than ca. 1000 domains active.

+---------- Defined Xenstore message types ----------

Nit: Odd 'ŕ' here.

Thanks for spotting.

Also the 1000 domains boundary is just an example aiui, so you may want
to add "e.g.".

The boundary is related to the amount of data returned by XS_DIRECTORY
for "/local/domain". As soon as the sum of (strlen(domid_string) + 1)
of all domains is larger than 4080 (XENSTORE_PAYLOAD_MAX - sizeof(struct
xsd_sockmsg)), XS_DIRECTORY won't be able to return the list of children
for "/local/domain". The limit will depend on the domid distribution:
in the worst case it can be 640 (all domids 5 digits), in the best case
it will be 1038 (all domids below 1000 present, the rest below 10000).

And elsewhere there can't be very many nodes resulting in a similar
situation?

Not without someone trying to force this by will (only possible by a
root user of dom0, e.g. via creating lots of nodes in the same directory
via "xenstore-write").

Having lots of domains is an "easy" way to create this scenario via
perfectly valid and sensible operations.

I'm further uncertain here regarding your use of hard tabs.

I'm following the style of the rest of the document, which is making use
of hard tabs as well. I can switch to spaces if you want.

I'm not demanding a change. I'd like it to be considered though that people
may in principle be using different tab-to-nr-of-blanks mappings, which
causes tabular arrangements like the above one to come out distorted.

Fair enough.

I'm going to switch to blanks for the table.


Juergen

OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

OpenPGP_signature.asc
Description: OpenPGP digital signature