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Re: [Qemu-devel] [Qemu-block] [PATCH v3 00/13] main-loop: Get rid of fd_

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v3 00/13] main-loop: Get rid of fd_read_poll and qemu_set_fd_handler2
Date: Tue, 19 May 2015 16:02:38 +0100
User-agent: Mutt/1.5.23 (2014-03-12)

On Tue, May 19, 2015 at 10:50:57AM +0000, Fam Zheng wrote:
> v3: Replace previous 13 with a simple return type conversion patch.
>     Drop RFC.
> This carries out the mandate in the comment of qemu_set_fd_handler2 and 
> removes
> fd_read_poll from the code base, because it will make the work easier to
> convert ppoll to epoll in main loop.  Also, the aio interface doesn't have a
> read poll callback, which means this conversion woule be necessary if we want
> to move iohandler fds from main loop to AioContext.
> There are five users of the read poll callback now: qemu-nbd, l2tpv3, netmap,
> socket and tap, which are all covered.
> Patch 1 adds a stub for qemu_set_fd_handler which will be referenced in coming
> patches.
> Patch 2 converts qemu-nbd which compares two global numbers in the 
> fd_read_poll
> callback.
> Patches 3~6 converts the four net devices, all of which checks
> qemu_can_send_packet() in the callback.
> Patch 7 and 8 finally removes the function.
> The rest patches are cleanup for the unuseful return value of
> qemu_set_fd_handler.
> Please review!

I looked at the first few patches.

The qemu-nbd.c conversion is good.  It finds locations where read_poll
state transitions happen and updates file descriptor monitoring there.

The l2tpv3 and netmap conversion throws away read_poll and introduces
problems instead.

Please look at the patches again and either:

1. Convert everything like you converted qemu-nbd.c.  This is a
   conservative approach and we can be confident that behavior is

2. Convert more carefully, making sure that packets are actually
   dropped and QEMU does not spin on a readable fd.

I prefer #1 because it's easier to check that behavior is still correct.
It also minimizes queuing inside QEMU.


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