Re: [Qemu-devel] [Qemu-block] [PATCH v3 00/13] main-loop: Get rid of fd_read_poll and qemu_set_fd_handler2

From: Stefan Hajnoczi <stefanha@gmail.com>
To: Fam Zheng <famz@redhat.com>
Cc: qemu-block@nongnu.org, Juan Quintela <quintela@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	qemu-devel@nongnu.org, Vincenzo Maffione <v.maffione@gmail.com>,
	"Vassili Karpov (malc)" <av1474@comtv.ru>,
	Gerd Hoffmann <kraxel@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Amit Shah <amit.shah@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Giuseppe Lettieri <g.lettieri@iet.unipi.it>,
	Luigi Rizzo <rizzo@iet.unipi.it>
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	[thread overview]
Message-ID: <20150519150238.GK9338@stefanha-thinkpad.redhat.com> (raw)
In-Reply-To: <1432032670-15124-1-git-send-email-famz@redhat.com>

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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
   unchanged.

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.

Stefan

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