Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction

[Thiemo Seufer]

Stefan Weil wrote:
> Thank you, Paul, for your explanation which clarified Thiemo's statement.
> 
> I now checked how my published test code could contribute to a DoS attack.
> 
> Current QEMU HEAD:
> 
> * The code "hangs" as I wrote before. This is from a user's point of view.
>   "Hanging" means, that the test process runs in an infinite loop using any
>   CPU time it can get in the virtual machine. QEMU uses all available
>   CPU time from the host CPU.

This is a bug in qemu, since it doesn't match CPU behaviour. While the
architecture spec claims UNPREDICTABLE, such a code sequence shouldn't
impede other processes on the same CPU. Throwing an RI exception should
suffice for the general case (i.e. not AR7).

>   With single stepping enabled or in the debugger, the test code won't
>   hang but give a random result.
> 
> Patched QEMU HEAD (see appended patch file):
> 
> * The code works in a well defined way. An optional message in the log file
>   will show the faulty statement. It won't amount to a DoS because it
>   is disabled by default.

Sorry, but I missed the "well defined". What does the jump in the branch
delay slot exactly _do_ now? Where does the PC point to when it was a
conditional branch which wasn't taken?

[snip]
> * show optional message when any branch bits in hflags are already set
>   before a branch instruction is generated (so we have a branch in the
>   delay slot)

Agreed on that, since it is debug output which is only written when
asked for.


Thiemo