Possible bug when setting aarch64 watchpoints

[Chris Howard]

Hi, I’m new to qemu (and even bug-reporting) so apologies in advance…

The MASK field in DBGWCRx_EL1 is **5** bits wide [28:24].

In target/arm/kvm64.c I found the line:

 wp.wcr = deposit32(wp.wcr, 24, 4, bits);               // ie **4** bits 
instead of **5**


If it’s not copying (or calculating?) the number of bits correctly this would 
explain the behaviour I’m seeing:

If I set

DBGWVR0_EL1 = 0x00800000

and

DBGWCR0_EL1 = 0x1F<<24 | 0xFF<<5 | 0b11<<3 | 0b11<<1 | 0b1<<0

and then access  memory [0x00807FFF]  I get a watchpoint exception. (ie. 
watchpoints ARE working/enabled)

But if I access [0x008080000] I *don’t* get an exception.

**If the MASK field gets set to 0b1111 instead of 0b11111 then only the bottom 
15 bits of the address get masked (instead of 31) and the masked address isn’t 
0x00800000, and the exception won’t be triggered.**


Unfortunately, changing the 4 to a 5 and recompiling had no effect :-(

I may well have misunderstood something. :-/

—Chris