Re: [Qemu-devel] Improve QEMU performance with LLVM codegen and other techniques

[Peter Maydell]

On 29 November 2011 07:03, 陳韋任 <address@hidden> wrote:
>
>  1. cpu_unlink_tb (exec.c)

This function is broken even for pure TCG -- we know it has a race condition.
As I said on IRC, I think that the right thing to do is to start
by overhauling the current TCG code so that it is:
 (a) properly multithreaded (b) race condition free (c) well documented
 (d) clean code
Then you have a firm foundation you can use as a basis for the LLVM
integration (and in the course of doing this overhaul you'll have
figured out enough of how the current code works to be clear about
where hooks for invalidating your traces need to go).

> The big problem is debugging.

Yes. In this sort of hotspot based design it's very easy to end up
with bugs that are intermittent or painful to reproduce and where
you have very little clue about which version of the code for which
address ended up misgenerated (since timing issues mean that what
code is recompiled and when it is inserted will vary from run to
run). Being able to conveniently get rid of some of this nondeterminism
is vital for tracking down what actually goes wrong.

-- PMM