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Re: [Qemu-devel] qemu virtIO blocking operation - question

From: Sinha, Ani
Subject: Re: [Qemu-devel] qemu virtIO blocking operation - question
Date: Mon, 12 Sep 2011 13:31:57 -0500

On Sep 11, 2011, at 6:34 AM, Stefan Hajnoczi wrote:

> On Fri, Sep 09, 2011 at 07:45:17PM -0500, Sinha, Ani wrote:
>> So I am writing a virtIO driver for a device that supports blocking calls 
>> like poll() etc. Now the front end paravirtualized driver mirrors the 
>> request to the backend "host" qemu process that then does the actual call on 
>> the host kernel on behalf of the guest. Now my question is, when I do a 
>> blocking call from within the callback that I have registered when I created 
>> the virtqueue, does it block the entire vcpu? If it does, do I have to 
>> create an async context for it? Looking at virtio-net and virtio-block, I 
>> can see the concept of bottom halves but I am not sure if this helps me in 
>> any way.
> What device are you adding?  It would help to share what you are trying
> to accomplish.

We are trying to paravirtualize the IPMI device (/dev/ipmi0).

> QEMU has an event loop where you can register non-blocking file
> descriptors.  Callback functions that are invoked when the file
> descriptor becomes readable/writable.  This is how block I/O,
> networking, VNC UI, monitor, etc work and probably what you should do
> too.

Hmm, I looked closely at virtio-block driver. It looks like it built on top of 
posix aio infrastructure (posix-aio-compat.c). I'll probably have to build 
something similar or use the event loop you suggested.

> You may find these posts I wrote helpful, they explain vcpu threads and
> the I/O thread:
> http://blog.vmsplice.net/2011/03/qemu-internals-big-picture-overview.html
> http://blog.vmsplice.net/2011/03/qemu-internals-overall-architecture-and.html

Thanks a lot. I will look into these articles.


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