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

[Sinha, Ani]

On Sep 28, 2011, at 1:51 AM, Stefan Hajnoczi wrote:

> 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
> easiest.

I need a helper thread to do blocking IO. The device IOCTLS are inherently 
blocking, unfortunately.

Ani


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