booke206_tlb_ways() returning 0

[Peter Maydell]

Hi; Coverity points out a possible issue in booke206_get_tlbm()
(this is CID 1421942):


static inline ppcmas_tlb_t *booke206_get_tlbm(CPUPPCState *env, const int tlbn,
                                              target_ulong ea, int way)
{
    int r;
    uint32_t ways = booke206_tlb_ways(env, tlbn);
    int ways_bits = ctz32(ways);
    int tlb_bits = ctz32(booke206_tlb_size(env, tlbn));
    int i;

    way &= ways - 1;
    ea >>= MAS2_EPN_SHIFT;
    ea &= (1 << (tlb_bits - ways_bits)) - 1;
    r = (ea << ways_bits) | way;

Here if booke206_tlb_ways() returns zero, then ways_bits()
will be 32 and the shift left 'ea << ways_bits' is undefined
behaviour.

My first assumption was that booke206_tlb_ways() is not supposed
to ever return 0 (it's looking at what I think are read-only
system registers, and it doesn't make much sense to have
a zero-way TLB). So I tried adding an assert:

diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
index 88d94495554..aedb6bcb265 100644
--- a/target/ppc/cpu.h
+++ b/target/ppc/cpu.h
@@ -2403,6 +2403,7 @@ static inline int booke206_tlb_ways(CPUPPCState
*env, int tlbn)
 {
     uint32_t tlbncfg = env->spr[SPR_BOOKE_TLB0CFG + tlbn];
     int r = tlbncfg >> TLBnCFG_ASSOC_SHIFT;
+    assert(r > 0);
     return r;
 }

However, this isn't right, because it causes one of the check-acceptance
tests to fail, with this backtrace:

#3  0x00007ffff074d412 in __GI___assert_fail (assertion=0x5555560a0d7d
"r > 0", file=0x5555560a0d40
"/home/petmay01/linaro/qemu-from-laptop/qemu/target/ppc/cpu.h",
line=2406, function=0x5555560a1720 <__PRETTY_FUNCTION__.20811>
"booke206_tlb_ways") at assert.c:101
#4  0x0000555555a9939b in booke206_tlb_ways (env=0x555556e52a30,
tlbn=2) at /home/petmay01/linaro/qemu-from-laptop/qemu/target/ppc/cpu.h:2406
#5  0x0000555555a9b3ac in mmubooke206_get_physical_address
(env=0x555556e52a30, ctx=0x7fffd77008a0, address=9223380835095282947,
rw=0, access_type=0, mmu_idx=1) at
/home/petmay01/linaro/qemu-from-laptop/qemu/target/ppc/mmu_helper.c:1093
#6  0x0000555555a9c25d in get_physical_address_wtlb
(env=0x555556e52a30, ctx=0x7fffd77008a0, eaddr=9223380835095282947,
rw=0, access_type=0, mmu_idx=1) at
/home/petmay01/linaro/qemu-from-laptop/qemu/target/ppc/mmu_helper.c:1455
#7  0x0000555555a9c82b in cpu_ppc_handle_mmu_fault
(env=0x555556e52a30, address=9223380835095282947, rw=0, mmu_idx=1)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/target/ppc/mmu_helper.c:1597
#8  0x0000555555a9f975 in ppc_cpu_tlb_fill (cs=0x555556e49560,
addr=9223380835095282947, size=1, access_type=MMU_DATA_LOAD,
mmu_idx=1, probe=false, retaddr=140735610345781) at
/home/petmay01/linaro/qemu-from-laptop/qemu/target/ppc/mmu_helper.c:3053
#9  0x00005555558e1422 in tlb_fill (cpu=0x555556e49560,
addr=9223380835095282947, size=1, access_type=MMU_DATA_LOAD,
mmu_idx=1, retaddr=140735610345781) at
/home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cputlb.c:1017
#10 0x00005555558e279b in load_helper (env=0x555556e52a30,
addr=9223380835095282947, oi=1, retaddr=140735610345781, op=MO_8,
code_read=false, full_load=0x5555558e2b3a <full_ldub_mmu>) at
/home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cputlb.c:1534
#11 0x00005555558e2b80 in full_ldub_mmu (env=0x555556e52a30,
addr=9223380835095282947, oi=1, retaddr=140735610345781)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cputlb.c:1624
#12 0x00005555558e2bb4 in helper_ret_ldub_mmu (env=0x555556e52a30,
addr=9223380835095282947, oi=1, retaddr=140735610345781)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cputlb.c:1630
#13 0x00007fff900fd9c6 in code_gen_buffer ()
#14 0x00005555558f9915 in cpu_tb_exec (cpu=0x555556e49560,
itb=0x7fff900fd780 <code_gen_buffer+1038163>)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cpu-exec.c:172
#15 0x00005555558fa732 in cpu_loop_exec_tb (cpu=0x555556e49560,
tb=0x7fff900fd780 <code_gen_buffer+1038163>, last_tb=0x7fffd7701078,
tb_exit=0x7fffd7701070) at
/home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cpu-exec.c:619
#16 0x00005555558faa4c in cpu_exec (cpu=0x555556e49560) at
/home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cpu-exec.c:732
#17 0x00005555558bcf29 in tcg_cpu_exec (cpu=0x555556e49560) at
/home/petmay01/linaro/qemu-from-laptop/qemu/cpus.c:1405
#18 0x00005555558bd77f in qemu_tcg_cpu_thread_fn (arg=0x555556e49560)
at /home/petmay01/linaro/qemu-from-laptop/qemu/cpus.c:1713
#19 0x0000555555f2ff3f in qemu_thread_start (args=0x555556e8dd10) at
/home/petmay01/linaro/qemu-from-laptop/qemu/util/qemu-thread-posix.c:519
#20 0x00007ffff0b156db in start_thread (arg=0x7fffd7704700) at
pthread_create.c:463
#21 0x00007ffff083e88f in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:95

So under what circumstances can booke206_tlb_ways()
validly return 0? Should booke206_get_tlbm() cope with a
zero return, or can it assert() that it will never
call booke206_tlb_ways() in a way that will cause one?

thanks
-- PMM