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Re: [Qemu-devel] The reason behind block linking constraint?

From: 陳韋任
Subject: Re: [Qemu-devel] The reason behind block linking constraint?
Date: Mon, 26 Sep 2011 18:49:30 +0800
User-agent: Mutt/1.5.21 (2010-09-15)

Hi, Max

  Sorry, I have to be sure what you talked about is guest or host.
Let me try.

> Well, my explanation sucks. Let's say it other way, more precisely:
> - you have two pieces of code in different pages, one of them jumps to the 
> other;

  guest code in different guest pages.

> - and you have two TBs, tb1 for the first piece and tb2 for the second;

  tb1 and tb2 are in the code cache (host binary).

> - and you link them and there's direct jump from tb1 to tb2;
> - now you change the mapping of the code page that contains second piece of 
> code;

  change the mapping of the guest page which contains second piece of
guest binary. Mapping guest page to what? Host virtual address?

> - after that there's another code (or no code at all) at the place where the 
> second piece of code used to be;
> - but the jump to tb2 still remains in tb1.

  there's another code (or no code at all) at the guest page which
used to contain second piece of guest binary.

  So if we execute tb2, it might have wrong memory access through
the mapping of guest page. Am I right?
> >   I assume that "all TBs in that page will be gone" means QEMU will
> > invalidate those TBs.
> No, it won't. I had to say "all code in that page will be gone", sorry for 
> the confusion.

  O.K., here TB is in the code cache, page is guest page, code is guest
binary. So the second piece of guest binary in that guest page will be
gone, but TBs related to the guest page still remain in the code cache.
No invalidation here.
> > If not, I think tb_find_fast will return tb2 which should not be executed.
> It won't either. tb_find_fast searches tb this way:
>   tb = env->tb_jmp_cache[tb_jmp_cache_hash_func(pc)];
> but 'page mapping change' implies TLB flush, at least for that page.
> Both tlb_flush and tlb_flush_page will clear env->tb_jmp_cache and 
> tb_find_fast will have to call tb_find_slow.

  Yes, I see QEMU use memset to clear env->tb_jmp_cache while doing
> Exactly. The exception will be raised inside the guest and the guest will 
> execute its page fault handler or whatever.

  Thanks, Max. Although I still doesn't totally understand how softmmu
is done in QEMU, but the whole picture is much clear to me now. And
about the page boundary restraint of (direct) block linking,

  if ((pc & TARGET_PAGE_MASK) == (tb->pc & TARGET_PAGE_MASK) ||
      (pc & TARGET_PAGE_MASK) == ((s->pc - 1) & TARGET_PAGE_MASK))  {

I guess this is because it'll be too complicated to track all links
jump to this (guest) page. A guest page might contains hundreds of TBs.
If the guest page is gone, then it's not a easy thing to do unlinking.
Does this make sense?


Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667

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