This test failure is caused by the kernel not setting dscr_inherit when
the user dscr MSR is written to. Which is caused by FSCR not raising a
facility unavailable interrupt. Which is ironic as one of the other
patches that is backported is 993e3d96fd08c3ebf7566e43be9b8cd622063e6d
("powerpc/64s: Don't let DT CPU features set FSCR_DSCR"), which should
prevent this situation.
The reason dscr_inherit_test does not fail is because it writes to the
kernel MSR first. Changing dscr_inherit_exec_test to do the same, it
passes.
Testing 5.8 shows that it's fixed somehow, though I haven't found the
exact commit that would fix this. Reverting
912c0a7f2b5daa3cbb2bc10f303981e493de73bd ("powerpc/64s: Save FSCR to
init_task.thread.fscr after feature init") seems reasonable here.
But failing to inherit dscr would cause cache performance regressions
that are hard to justify respining the kernel for.
Cascardo.
** Changed in: linux (Ubuntu Eoan)
Status: Confirmed => Won't Fix
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https://bugs.launchpad.net/bugs/1888332
Title:
[Potential Regression] dscr_inherit_exec_test from powerpc in
ubuntu_kernel_selftests failed on B/E/F
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