powerpc/64s: system call rfscv workaround for TM bugs

powerpc/64s: system call rfscv workaround for TM bugs

The rfscv instruction does not work correctly with the fake-suspend mode
in POWER9, which can end up with the hypervisor restoring an incorrect
checkpoint.

Work around this by setting the _TIF_RESTOREALL flag if a system call
returns to a transaction active state, causing rfid to be used instead
of rfscv to return, which will do the right thing. The contents of the
registers are irrelevant because they will be overwritten in this case
anyway.

Fixes: 7fa95f9adaee7 ("powerpc/64s: system call support for scv/rfscv instructions")
Reported-by: Eirik Fuller <efuller@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210908101718.118522-1-npiggin@gmail.com