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Re: [Qemu-devel] Time complexity for self-modifying code
From: |
Paul Brook |
Subject: |
Re: [Qemu-devel] Time complexity for self-modifying code |
Date: |
Thu, 8 Feb 2007 20:37:36 +0000 |
User-agent: |
KMail/1.9.5 |
On Thursday 08 February 2007 20:01, Michael Gagnon wrote:
> Hello. I'm a student at George Mason University and I had a question
> regarding the time complexity of QEMU's algorithm for dealing with
> self-modifying code.
>
> From looking at the QEMU Internals documentation
> (http://fabrice.bellard.free.fr/qemu/qemu-tech.html), it seems that
> QEMU's method for handling self-modifying code might have different
> algorithmic efficiency classes for it's average case and worst case. As
> in, on average I assume that QEMU emulates instructions at O(n)
> efficiency. In the worst-case, might self-modifying code change the
> efficiency of QEMU to another order of efficiency, such as O(n^2)? Any
> thoughts would be greatly appreciated. Thanks!
Depends what your N is.
Worst case for SMC (Self Modifying Code) is modifying code in the same TB
(Translation Block) as the store instruction. This kind of fault requires
O(tb_size) time, so executing a TB (assuming every insn traps) takes
O(tb_size ^2) time. However the page boundaries impose a hard limit on the
size of a TB.
Thus for N < TARGET_PAGE_SIZE worst case total execution time is O(N^2), but
for N > TARGET_PAGE_SIZE total execution time is still O(N).
For SMC the constant factor may be orders of magnitude larger than for regular
code.
Paul