Re: [Qemu-devel] How does TCG gen host code for a TB?

[Peter Maydell]

On 11 December 2015 at 03:43, Valerón JC <address@hidden> wrote:
> I want to trace a bug in tcg, which for me, at some point, generate infinite
> loop TB chains, that's unexpected. and I've found the final TB(head, since
> they're chaining) which run in an infinite loop, and I know a very weird
> trick to 'disable' this bug, so I would like to track the tcg-ops for the
> TB, hope that I can figure what's wrong.

If the guest code is an infinite loop then we will generate
a chain of TBs which goes round in a loop too. (Execution will
escape from the loop via longjmp when there is a guest interrupt
or other exception.)

> but when I read the tcg_gen_code(), I'm confused...

> how does tcg_gen_code() generate codes for one TB? if I read the code
> correctly, gen_intermediate_code() will not flush the
> tcg_ctx->gen_opc_buf[], codes for previous TB are mixed together... and
> tcg_gen_code() will start gen from index 0 of gen_opc_buf, how does it
> generate codes for the TB just created?

This code has changed, and tcg_ctx->gen_opc_buf doesn't exist any more.
We store ops in a linked list now rather than an array.
The answer to your question in general is still the same, though:
before calling gen_intermediate_code() we call tcg_func_start(),
which resets the TCGContext to a clean state, including "no temporaries
allocated", "no labels" and "no ops".

thanks
-- PMM