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Re: [PATCH v1] xen/domctl: make domctl_lock generic


  • To: Penny Zheng <Penny.Zheng@xxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Wed, 23 Jul 2025 12:07:54 +0200
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  • Cc: ray.huang@xxxxxxx, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • Delivery-date: Wed, 23 Jul 2025 10:08:17 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 23.07.2025 08:53, Penny Zheng wrote:
> Not only domctl-op could do foreign updates to guest state, some hypercall,
> like HVMOP_set_param, could also do, and they all need domctl_lock for
> syncronization.

What's "all" here? HVMOP_set_param is the sole such case, and hence - aiui -
it uses the domctl lock only to lock out actual domctl-s. Of which there are
not going to be any with DOMCTL=n, ...

> Later, we will introduce CONFIG_DOMCTL to wrap domctl.c. In order to
> continue using domctl_lock when CONFIG_DOMCTL not defined, we'd like to move
> domctl_lock_acquire/release() out of domctl.c, and into more common space,
> domain.c

... as you indicate is where we're going to move to as a possibility. Hence
the domctl lock is meaningless without DOMCTL=y. At which point moving it out
of domctl.c is the wrong course of action. With DOMCTL=n, we'll rather need
stub domctl_lock_{acquire,release}(), which then likely would do nothing (or
alternatively "#ifdef CONFIG_DOMCTL" could be put around the two uses in
hvm_set_param()).

> The movement could also fix CI error of a randconfig picking both
> PV_SHIM_EXCLUSIVE=y and HVM=y results in hvm.c being built, but
> domctl.c not being built, which leaves domctl_lock_acquire/release()
> undefined, causing linking to fail.

If the work towards DOMCTL as a Kconfig control can't be progressed pretty
quickly (and I don't expect that to be much faster than the SYSCTL work),
then I expect our prime option (apart from reverting the earlier change) is
to accept a bunch of dead code in the shim binary again, by moving domctl.o
out of the PV_SHIM_EXCLUSIVE common/Makefile section as well (plus whatever
other adjustments are necessary to undo that earlier dead code elimination).

Jan



 


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