Hi Juergen,
On 28/07/2023 14:24, Juergen Gross wrote:
On 28.07.23 14:48, Julien Grall wrote:
Hi,
On 28/07/2023 13:06, Juergen Gross wrote:
On 28.07.23 13:19, Julien Grall wrote:
In case of a runtime check I
agree that a more central place would be preferred.
In the end I don't mind that much, but
BUILD_BUG_ON(XENSTORE_PAYLOAD_MAX >=
(typeof((struct node_hdr *)NULL->datalen))(-1));
is a little bit clumsy IMHO.
Agree. We could introduce FIELD_SIZEOF() (as Linux did) to hide
the complexity. The code would then look like:
>= (8 * FIELD_SIZEOF(struct node_hdr, datalen))
Oh, I guess you mean sizeof_field().
And even with that it would look quite clumsy:
BUILD_BUG_ON(XENSTORE_PAYLOAD_MAX >=
(1UL << (8 * sizeof_field(struct node_hdr, datalen))));
How about keeping the BUILD_BUG_ON() in write_node_raw() and add the
following comment on top of handle_input():
Some fields in Xenstored are sized based on the max payload (see
various BUILD_BUG_ON()). This would need extra runtime check if we
ever decide to have a dynamic payload size.
I _could_ do that, but where to stop adding such comments?
When someone other than the author is able to understand the code
without too much effort. More comments never hurts, less will in the
longer run (see below).
I agree with that statement in general, but requesting a comment to aid a
future potential change violating the Xenstore wire protocol is a little
bit
weird.
Well... This is violating the existing protocol, but it is not set in
stone and I think this is acceptable to update it when there is no
change for the VMs and for new features (e.g. Live-Update/Live-Migration).
TBH, I really don't see the point doing that.
In case a patch came up upstream trying to violate
XENSTORE_PAYLOAD_MAX I would
surely NACK it.
That's assuming you will still be around when this happens :). I am
not wishing anything bad but the code will likely outlast any of us.
Maybe. But would you really Ack patches adding comments like that in other
areas?
Potentially yes. We had a similar discussion on Arm when allowing
paddr_t to be 32-bit.
[...]
In case we need payloads larger than XENSTORE_PAYLOAD_MAX we should
split the
related operation in multiple parts (see e.g. XS_DIRECTORY_PART or
XS_CONTROL
for uploading a new kernel to Xenstore-stubdom for live update).
Which is, BTW,
the way AWS should have handled the migration problem (transactions
come to my
mind in this context).
I wasn't part of the original design, but I can see why it was done
like that.
I can see why it was done that way, but this doesn't mean I can understand
why such a design should be supported by adding comments helping to
repeat such
a bad decision.
Using multiple commands has also its downside. The first that comes to
my mind if that you need to keep around the data. But, with your
proposal, you we wouldn't be able to store it in the database (like
for transaction update) as datalen can only be 65KB.
I wasn't aware that a complete transaction needs to be kept in a single
data
base record. :-)
IIUC, you are thinking that the client will restore all the state bits
by bits. But if you look at the design in
docs/designs/xenstore-migration.md, this is a blob.
It would work perfectly fine to allocate the needed memory via talloc()
and to
reference it from a special node being part of the transaction, or to
not use
a node at all (see again the XS_CONTROL example).
I am not convinced the complexity is worth it here. To be honest, I
think the payload limit should have been relaxed for Live-Update as well
as you don't gain much to split. That said, this is less a concern
because you are not time constrained.
[...]
But maybe that comment was based on wrong assumptions, like the mentioned
change not violating the protocol. >
I am happy to rewrite the comment so it doesn't lead to think that you
(as the maintainer) are open to have a more relax length check.
Yes, please make a suggestion for a proper comment not suggesting we are
fine
to violate the wire protocol.
Here we go:
"The payload size is not only currently restricted by the protocol but
also the internal implementation (see various BUILD_BUG_ON())."
--
Julien Grall
