Re: [Qemu-devel] TCG register allocator

[Peter Maydell <peter.maydell@linaro.org>]

On 25 January 2012 19:10, Xin Tong <xerox.time.tech@gmail.com> wrote:
Peter Maydell wrote:
>> cpu_restore_state() calls gen_intermediate_code_pc() to
>> request a retranslation of the TB with extra info to allow
>> us to do a host-PC-to-guest-PC lookup
>>  * Note that gen_intermediate_code_pc() overwrites the generated
>> code that already exists in memory, and stops as soon as it
>> reaches the point of the exception. This is harmless because
>> we are just rewriting the same bytes to memory that were there
>> already, but disastrous if...

> in the case you described above, which code guarantees that the
> re-generated code is laid down in the place where the old code is.
> This has to be do somewhere in tcg.

Actually, it's not in TCG but in the bit of QEMU which calls TCG.

When we first translate a block of code we decide where its
generated code will sit (in exec.c:tb_gen_code()) and set tb->tc_ptr
to point at this location. Then when translate-all.c:cpu_gen_code()
does this first translation it passes tcg_gen_code() a gen_code_buf
argument which is this tb->tc_ptr.
Later if we need to do a restore, translate-all.c:cpu_restore_state()
calls tcg_gen_code_search_pc(), and again it passes a gen_code_buf
argument for where generated code should be written. It uses tb->tc_ptr
for the TranslationBlock, so it will always be at the same place.

-- PMM