Hello developers and developer-adjacents!
With the impending integration of illumos#13915, the private-but-used-by-others
installctx() call gains a 9th parameter, which is either NULL for old-behavior,
or a non-NULL result of the new installctx_preallocate() call for installctx()
callers which are in a kpreempt_disable() block. This constitutes a FLAG DAY
for users of installctx(), and their outside-illumos-gate modules that call
installctx() should be rebuilt on a kernel that has illumos#13915 in it.
Of general interest is that illumos-available HVM hypervisors like: bhyve, KVM,
and VirtualBox use installctx(), and will have to take into account this
change. A pre-this-flag-day binary will have undefined behavior when run in a
post-this-flag-day kernel. I know at least OmniOS has changes ready for their
build of VirtualBox, and I encourage other distros to do the same.
- If the installctx() call is inside a kpreempt_disable() section (like it is
in illumos-kvm), the caller must use installctx_preallocate() in advance of
kpreempt_disable(), and pass its result as the new 9th argument to
installctx(). For example:
struct ctxop *ctx;
...
ctx = installctx_preallocate();
...
kpreempt_disable();
installctx(....., ctx);
- If the installctx() call is NOT in a kpreempt_disable() section, an additonal
NULL must be added at the end of the parameter list for installctx(). An
example diff:
- installctx(...);
+ installctx(..., NULL);
Since installctx() is not a documented part of the DDI, we can make this change
with only this warning.
Of note: There is an ADDITIONAL change coming (illumos#13917) that refactors
installctx() entirely to be more "ergonomic" as the filer accurately puts it.
And that may too require another flag day.
I apologize for the need for a flag day, and appreciate your patience. I
encourage people to review illumos#13917, which will refine thread
context-switch hooks even further, and lend itself well to potentially reducing
memory allocations by encouraging reuse in some situations.
Thanks,
Dan
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