Re: [Qemu-devel] throwing away translated code on CPU reset

[陳韋任]

On Thu, Jan 12, 2012 at 02:00:38PM +0000, Peter Maydell wrote:
> When doing TCG code translation, the target-foo translate.c
> code is allowed to bake assumptions into the generated code from
> the current values of various fields in the CPUState. This then
> imposes the requirement that if the field is changed then tb_flush
> must be called to throw away the now-incorrect generated code.
> 
> However, cpu_reset() changes (unsurprisingly) lots of fields in
> the CPUState, but it doesn't call tb_flush()...

  I dig what tlb_flush does further and think maybe we don't need to call 
tb_flush
when tlb_flush is called.

  First, look at tlb_flush (exec.c). It clears env's tb_jmp_cache which use GHA
as an index to search if there is a translated code. Since tb_jmp_cache is reset
now, QEMU is forced to call tb_find_slow which uses GPA as the index.

  In tb_find_slow's for loop, it compares hit TranslationBlock's various fields
with current values. To be more specific,

static TranslationBlock *tb_find_slow(...) {

    for(;;) {
        tb = *ptb1;
        if (!tb)
            goto not_found;
        if (tb->pc == pc &&                    <--- Here
            tb->page_addr[0] == phys_page1 &&
            tb->cs_base == cs_base &&
            tb->flags == flags) {
        }
   }
}

  What do you think?

Regards,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667
Homepage: http://people.cs.nctu.edu.tw/~chenwj