Comment 7 for bug 1647683

Peter Maydell <email address hidden> writes:

> On 6 December 2016 at 14:51, Alex Bennée <email address hidden> wrote:
>> A bug (1647683) was reported showing a crash when removing
>> breakpoints. The reproducer was bisected to 3359baad when tb_flush was
>> finally made thread safe. While in MTTCG the locking in
>> breakpoint_invalidate should have prevented any problems currently
>> tb_lock() is a NOP for system emulation.
>>
>> On top of it all the invalidation code was special-cased between user
>> and system emulation which was ugly. As we now have a safe tb_flush()
>> we might as well use that when breakpoints and added and removed. This
>> is the same thing we do for single-stepping and as this is during
>> debugging it isn't a major performance concern.
>>
>> Reported-by: Julian Brown
>> Signed-off-by: Alex Bennée <email address hidden>
>> ---
>> exec.c | 31 ++++---------------------------
>> 1 file changed, 4 insertions(+), 27 deletions(-)
>>
>> diff --git a/exec.c b/exec.c
>> index 3d867f1..e991221 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -685,29 +685,6 @@ void cpu_exec_realizefn(CPUState *cpu, Error **errp)
>> }
>>
>> #if defined(CONFIG_USER_ONLY)
>> -static void breakpoint_invalidate(CPUState *cpu, target_ulong pc)
>> -{
>> - mmap_lock();
>> - tb_lock();
>> - tb_invalidate_phys_page_range(pc, pc + 1, 0);
>> - tb_unlock();
>> - mmap_unlock();
>> -}
>> -#else
>> -static void breakpoint_invalidate(CPUState *cpu, target_ulong pc)
>> -{
>> - MemTxAttrs attrs;
>> - hwaddr phys = cpu_get_phys_page_attrs_debug(cpu, pc, &attrs);
>> - int asidx = cpu_asidx_from_attrs(cpu, attrs);
>> - if (phys != -1) {
>> - /* Locks grabbed by tb_invalidate_phys_addr */
>> - tb_invalidate_phys_addr(cpu->cpu_ases[asidx].as,
>> - phys | (pc & ~TARGET_PAGE_MASK));
>> - }
>> -}
>> -#endif
>> -
>> -#if defined(CONFIG_USER_ONLY)
>> void cpu_watchpoint_remove_all(CPUState *cpu, int mask)
>>
>> {
>> @@ -839,7 +816,7 @@ int cpu_breakpoint_insert(CPUState *cpu, vaddr pc, int flags,
>> QTAILQ_INSERT_TAIL(&cpu->breakpoints, bp, entry);
>> }
>>
>> - breakpoint_invalidate(cpu, pc);
>> + tb_flush(cpu);
>>
>> if (breakpoint) {
>> *breakpoint = bp;
>> @@ -855,6 +832,7 @@ int cpu_breakpoint_remove(CPUState *cpu, vaddr pc, int flags)
>> QTAILQ_FOREACH(bp, &cpu->breakpoints, entry) {
>> if (bp->pc == pc && bp->flags == flags) {
>> cpu_breakpoint_remove_by_ref(cpu, bp);
>> + tb_flush(cpu);
>> return 0;
>> }
>> }
>> @@ -865,9 +843,6 @@ int cpu_breakpoint_remove(CPUState *cpu, vaddr pc, int flags)
>> void cpu_breakpoint_remove_by_ref(CPUState *cpu, CPUBreakpoint *breakpoint)
>> {
>> QTAILQ_REMOVE(&cpu->breakpoints, breakpoint, entry);
>> -
>> - breakpoint_invalidate(cpu, breakpoint->pc);
>> -
>> g_free(breakpoint);
>> }
>>
>> @@ -881,6 +856,8 @@ void cpu_breakpoint_remove_all(CPUState *cpu, int mask)
>> cpu_breakpoint_remove_by_ref(cpu, bp);
>> }
>> }
>> +
>> + tb_flush(cpu);
>> }
>>
>> /* enable or disable single step mode. EXCP_DEBUG is returned by the
>
> I think this is the wrong direction. We only need to invalidate
> the TBs for the specific location the breakpoint is at.
> Even if we for the moment want to apply a big hammer of doing
> a full tb flush on breakpoint to fix this issue for this release,
> we shouldn't drop the indirection through breakpoint_invalidate(),
> because then we can't go back to invalidating just the parts we need
> easily later.

Why would we? It's not like fresh code generation is particularly slow,
especially in a debugging situation when you've just stopped the
machine.

The self-modifying-code paths make more sense to be partial in their
invalidations although I've never really measured quite how pathalogical
running a JIT inside QEMU is.

--
Alex Bennée