Re: [Qemu-devel] [PATCH] target-*: Get rid of "PC advancement" trick

[Richard Henderson <rth@twiddle.net>]

On 12/10/2015 10:47 AM, Sergey Fedorov wrote:
> The "PC advancement" trick was used just after recognizing that a
> breakpoint exception was going to be generated. This trick has had two
> points:
>   1. Guarantee that tb->size isn't zero: there are many places where it's
>      expected to be non-zero. In fact, that is even stated in the comment
>      for this field.
>   2. Try to satisfy disassembler's check for instruction length. To this
>      end, PC advancement was done for estimated instruction length, but
>      actually, didn't work properly in variable-instruction-length cases.
>
> Substitute this trick with checking for TB size at the end of
> translation. If we get an empty TB then just set tb->size to 1 and skip
> disassembling. Setting tb->size to 1 is enough to get correct behaviour,
> whereas an empty TB doesn't obviously need to be disassembled.

This doesn't help when the TB already has instructions, the TB would ordinarily 
cross a page boundary, and the breakpoint is at the page boundary.


r~