[RFC PATCH] exec: flush the whole TLB if a watchpoint crosses a page boundary

["Alex Bennée" <alex.bennee@linaro.org>]

There is no particular reason why you can't have a watchpoint in TCG
that covers a large chunk of the address space. We could be clever
about it but these cases are pretty rare and we can assume the user
will expect a little performance degradation.

NB: In my testing gdb will silently squash a watchpoint like:

  watch (char[0x7fffffffff]) *0x0

to a 4 byte watchpoint. Practically it will limit the maximum size
based on max-value-size. However given enough of a tweak the sky is
the limit.

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 exec.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/exec.c b/exec.c
index 5162f0d12f9..851ac180fe7 100644
--- a/exec.c
+++ b/exec.c
@@ -1056,7 +1056,11 @@ int cpu_watchpoint_insert(CPUState *cpu, vaddr addr, vaddr len,
         QTAILQ_INSERT_TAIL(&cpu->watchpoints, wp, entry);
     }
 
-    tlb_flush_page(cpu, addr);
+    if (((addr + len) & TARGET_PAGE_MASK) != (addr & TARGET_PAGE_MASK)) {
+        tlb_flush(cpu);
+    } else {
+        tlb_flush_page(cpu, addr);
+    }
 
     if (watchpoint)
         *watchpoint = wp;
-- 
2.20.1