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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 1/8] tcg: Clean up direct block chaining data fields
Date: Thu, 24 Mar 2016 16:11:22 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

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.


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