Paolo Bonzini <pbonz...@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
