Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields

["Alex Bennée" <alex.bennee@linaro.org>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 24/03/2016 16:01, Alex Bennée wrote:
>>>> >> OK I found that tricky to follow. Where does the value of the pointer
>>>> >> come from that sets these bottom bits? The TB jumping to this TB sets it?
>>
>> Where I get confused it what is the point of jmp_list_first? If these
>> are two circular lists do we care which the first in the list is? The
>> exit condition when coming out of searching seems when ntb with index =
>> orig tb with index.
>
> Say you have a list for blocks that jump to TB. The next pointer is in
> jmp_list_next[0] for blocks whose first jump is to TB. It is in
> jmp_list_next[1] for blocks whose second jump is to TB.
>
> However, because it is a circular list, you also need TB itself to be a
> part of the list. For TB, the next pointer is in jmp_list_first.
>
> Because TB probably doesn't jump to itself, the first link of the list
> of blocks that jumps to TB is not in jmp_list_next[].  Thus QEMU places
> it in tb->jmp_list_first.
>
> Say you have three tbs.  TB1's first jump and TB2's second jump lead to
> TB0.  Then the list starting at tb0->jmp_list_first goes like this:
>
>     tb0->jmp_list_first = tb1 | 0;
>       .--------------------'    |
>      |                 .--------'
>     tb1->jmp_list_next[0] = tb2 | 1;
>       .--------------------'      |
>       |                 .---------'
>     tb2->jmp_list_next[1] = tb0 | 2;
>
> There is also a case where a TB jumps to itself; it then appears twice
> in the list with different values in the low bits, such as this:
>
>     tb->jmp_list_first = tb | 0;
>      .--------------------'   |
>      |                .-------'
>     tb->jmp_list_next[0] = tb | 2;
>
> Other blocks jumping to TB would appear in the same list, too, either
> before or after the tb|0 link.

Right I follow now. Extra ascii art always helps ;-)

--
Alex Bennée