Commit 70fe3d9 "powerpc: Restore FPU/VEC/VSX if previously used" introduces a
call to restore_math() late in the syscall return path, after MSR_RI has been
cleared. The MSR_RI flag is used to indicate whether the kernel can take
another exception or not. A cleared MSR_RI flag indicates that the kernel
cannot.
Unfortunately when a machine is under SLB pressure an SLB miss can occur
in restore_math() which (with MSR_RI cleared) leads to an unrecoverable
exception.
Unrecoverable exception 4100 at c0000000000088d8
cpu 0x0: Vector: 4100 at [c0000003fa473b20]
pc: c0000000000088d8: .load_vr_state+0x70/0x110
lr: c00000000000f710: .restore_math+0x130/0x188
sp: c0000003fa473da0
msr: 9000000002003030
current = 0xc0000007f876f180
paca = 0xc00000000fff0000 softe: 0 irq_happened: 0x01
pid = 1944, comm = K08umountfs
[link register ] c00000000000f710 .restore_math+0x130/0x188
[c0000003fa473da0] c0000003fa473e30 (unreliable)
[c0000003fa473e30] c000000000007b6c system_call+0x84/0xfc
The clearing of MSR_RI is actually an optimisation to avoid multiple MSR
writes, what must be disabled are interrupts. See comment in entry_64.S:
/*
* For performance reasons we clear RI the same time that we
* clear EE. We only need to clear RI just before we restore r13
* below, but batching it with EE saves us one expensive mtmsrd call.
* We have to be careful to restore RI if we branch anywhere from
* here (eg syscall_exit_work).
*/
At the point of calling restore_math() r13 has not been restored, as such, the
quick fix of turning MSR_RI back on for the call to restore_math() will
eliminate the occurrence of an unrecoverable exception.
We'd like to do a better fix in future.
Fixes: 70fe3d980f ("powerpc: Restore FPU/VEC/VSX if previously used")
Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
6e669f085d