Triaging old bug tickets... can you still reproduce this issue with the
latest version of QEMU? Or could we close this ticket nowadays?
** Changed in: qemu
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/1128935
Title:
MIPS r4k "TLB modified exception" generated for TLB entries that are
not visible to the TLBP instruction
Status in HelenOS branches:
New
Status in QEMU:
Incomplete
Bug description:
I occasionally see that the TLBP instruction fails to find the
corresponding TLB entry in the TLB Modified exception handler. This
behavior is unexpected, because the invocation of the TLB Modified
exception suggests there indeed is such an entry in the TLB and only
requires its dirty bit to be set.
The operating system which can trigger and is susceptible to this
behavior is a HelenOS branch located in lp:~jakub/helenos/mips-malta.
The QEMU version on which this is reproducible is QEMU 1.4.0 and also
some others.
When I looked into the QEMU sources, I noticed the following
discrepancy, which could potentially explain the behavior:
65 /* MIPS32/MIPS64 R4000-style MMU emulation */
66 int r4k_map_address (CPUMIPSState *env, hwaddr *physical, int *prot,
67 target_ulong address, int rw, int access_type)
68 {
<snip>
72 for (i = 0; i < env->tlb->tlb_in_use; i++) {
1865 void r4k_helper_tlbp(CPUMIPSState *env)
1866 {
<snip>
1875 for (i = 0; i < env->tlb->nb_tlb; i++) {
From the above it appears as if the the code which searches the TLB
for a matching entry searched also the QEMU-specific "shadow" TLB
entries, which is, however, not in line with how the TLBP instruction
searches the TLB. So if a matching entry is found on index >=
tlb_in_use, the HelenOS exception handler using TLBP to locate the
entry would hit an assertion on seeing the Index register bit P set.
I also suspect there is a similar issue with the TLB Invalid
exception, but thanks to the specifics of the MIPS 4Kc CPU, HelenOS is
not susceptible in this case.
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