Re: [Qemu-devel] Question about the block linking limitation

[Max Filippov]

>  Could someone help me to confirm that I understand how the QEMU cross page
> boundary checking correctly or not? Below is the source code I am looking at,
>
> static inline void gen_goto_tb(DisasContext *s, int tb_num, target_ulong eip)
> {
>
>    if ((pc & TARGET_PAGE_MASK) == (tb->pc & TARGET_PAGE_MASK) ||        --- 
> (1)
>        (pc & TARGET_PAGE_MASK) == ((s->pc - 1) & TARGET_PAGE_MASK))  {  --- 
> (2)
>    }
>
> }
>
> (a)                                (b)
>   tb->pc -->  ________               tb->pc -->  ________
>              |        |                         |        |
>              |        |                      -----------------
>              |        |                         |        |
>              |________|               s->pc --> |________|
>
>           ----------------                   -----------------
>               ________                           ________
>              |        |                         |        |
>       pc --> |        |                  pc --> |        |
>              |        |                         |        |
>              |________|                         |________|
>
>
>  My understanding is, if tb itself doesn't cross guest page boundary (a), then
> condition (1) is enough to check if the jump target (pc) is in the same guest
> page as tb is. Or, tb itself spans two guest pages (b), then we have to use
> condition (2) to check if the jump target (pc) is in the same guest page as tb
> is. In summary, those check (1) and (2) are used to avoid block linking to 
> cross
> guest page boundary.
>
>  Am I right? If so far so good, I am curious about why we need (s->pc - 1)
> instead of just (s->pc). Could you shed some light on that? Thanks in advance!

s->pc is updated each time a byte of code is fetched, when an instruction
is fully decoded s->pc points to the first byte of the next instruction. I see
that it only makes difference when a branching instruction ends exactly at
a page boundary. In this case (s->pc - 1) prevents from linking to the next
page, which seems to be its main purpose.

-- 
Thanks.
-- Max