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Re: [Qemu-devel] virtqueue corruption in emulation mode?

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] virtqueue corruption in emulation mode?
Date: Wed, 28 Sep 2011 09:51:39 +0100

On Wed, Sep 28, 2011 at 3:01 AM, Sinha, Ani <address@hidden> wrote:
> On Sep 27, 2011, at 12:17 AM, Stefan Hajnoczi wrote:
>> On Mon, Sep 26, 2011 at 07:16:56PM -0500, Sinha, Ani wrote:
>>> I am using the virtqueue (virtqueue_pop, virtqueue_push etc) in the 
>>> emulated mode (non-kvm mode) from an IO thread (a separate thread different 
>>> from main QEMU thread). What I am observing is that the virtqueue memory 
>>> seems to get corrupt. Either qemu crashes while performing virtqueue_push() 
>>> (virtqueue_push() -> virtqueue_fill() 
>>> ->bring_used_idx()->lduw_phys()->qemu_get_ram_ptr()->"bad ram offset") or 
>>> crashes when the guest accesses a bad memory while using virtqueue. Now 
>>> this never ever happens when I run QEMU in KVM mode (/dev/kvm present) OR 
>>> when I use my functions from within the main qemu thread. I am unable to 
>>> figure out why this is happening. I have looked into my code over and over 
>>> again and I can't seem to explain this behavior. Can any of you guys give 
>>> me any inkling?
>> QEMU is not thread-safe in general.  It uses a big lock to protect most
>> of its internal state.
> I see. So may be I should do something like qemu_set_fd_handler(fd, …) where 
> fd is a pipe and the handler does the virtqueue_push() etc?
> Now my question is, is it safe to do elem = virtqueue_pop(vq) from main event 
> loop, then so some work on the elem popped out in an worker thread and then 
> at some later point do a virtqueue_push(vq, elem) from that handler (which is 
> called by main_loop() ->main_loop_wait())?  In other words, the vq reference 
> is being used from the main event loop at two different points from two 
> different functions but not in a contiguous fashion within the same function.

Yes but do you need a helper thread?  Most of QEMU is based on
qemu_set_fd_handler() and callbacks, including for host network and
disk I/O.  If you follow the way QEMU does things it should be


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