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[Qemu-devel] When do we need to do TB unchaining?

From: 陳韋任
Subject: [Qemu-devel] When do we need to do TB unchaining?
Date: Thu, 29 Sep 2011 15:55:25 +0800
User-agent: Mutt/1.5.21 (2010-09-15)

Hi, all

  I am looking for when TB unchaining is needed. Currently, I
can only see there are three spots (take i386-softmmu as an

1. cpu_interrupt:

  When virtual devices raise interrupts, eventually apic_local_deliver
(apic.c) will call cpu_interrupt. cpu_interrupt will set up
env->interrupt_request, then call cpu_unlink_tb to unlink env's

  Here I have a question. I though cpu_interrupt is only used
in system mode to deliever virtual devices' interrupt. But it
seems process mode also has cpu_interrupt.

  I have ran some small programs in process mode under GDB, but
I never see cpu_interrupt is called. Do I have to run something
bigger to see when cpu_interrupt is called in process mode? Or
cpu_interrupt in process mode is only used in some rare cases?

2. cpu_exit:

  QEMU will register a host SIGALRM handler, host_alarm_handler
(qemu-timer.c),  when initialize the enviroment in system mode.
Then when host OS delivers SIGALRM to QEMU, host_alarm_handler
calls qemu_notify_event -> cpu_exit. cpu_exit raise env->exit_request
, then call cpu_unlink_tb to unlink env's TBs.

  There are other places where cpu_exit is called, like

  - cpu_signal: I think this is used when IOTHREAD is enabled.

  - DMA_init: I guess cpu_exit is called when DMA is done so
              that control is gave back to QEMU from the code

  - gdb_do_syscall: Don't know when it get called.

  - vm_stop -> cpu_stop_current: Don't know when it get called.

3. tb_phys_invalidate:

  QEMU will invalidate TBs related to a guest page which is
done by tb_invalidate_phys_page_range (exec.c), then 
tb_invalidate_phys_page_range calls tb_phys_invalidate to
invalidate a TB and unlink links to the TB.

  Please correct me if I am wrong or something miss. Thanks! 


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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