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Re: [Qemu-devel] [libvirt] [PATCH] qemu: Fix shutdown regression

From: Eric Blake
Subject: Re: [Qemu-devel] [libvirt] [PATCH] qemu: Fix shutdown regression
Date: Tue, 20 Sep 2011 13:03:16 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110906 Fedora/3.1.14-1.fc14 Lightning/1.0b3pre Mnenhy/0.8.3 Thunderbird/3.1.14

On 09/20/2011 12:52 PM, Anthony Liguori wrote:
On 09/20/2011 01:01 PM, Eric Blake wrote:
On 09/20/2011 11:39 AM, Jiri Denemark wrote:
The commit that prevents disk corruption on domain shutdown
(96fc4784177ecb70357518fa863442455e45ad0e) causes regression with QEMU
0.14.* and 0.15.* because of a regression bug in QEMU that was fixed
only recently in QEMU git. With affected QEMU binaries, domains cannot
be shutdown properly and stay in a paused state. This patch tries to
avoid this by sending SIGKILL to 0.1[45].* QEMU processes. Though we
wait a bit more between sending SIGTERM and SIGKILL to reduce the
possibility of virtual disk corruption.
src/qemu/qemu_capabilities.c | 7 +++++++
src/qemu/qemu_capabilities.h | 1 +
src/qemu/qemu_process.c | 19 +++++++++++++------
3 files changed, 21 insertions(+), 6 deletions(-)

ACK. But it would be nice if upstream qemu could give us a more reliable
indication of whether the qemu SIGTERM bug is fixed, so that we don't
data on a patched 0.14 or 0.15 qemu.

Can you be a lot more specific about what bug you mean?


That is, as part of fixing the bug in qemu,
we should also update -help text or something similar, so that libvirt
can avoid
making decisions solely on version numbers.

The version number *is* the right way to make decisions. We've gone
through this dozens of times.

The fact that distros backport all sorts of stuff means that you need to
maintain a matrix of versions with features. It's not our (upstream
QEMU's) responsibility to tell you the differences that exist in forks
of QEMU.

Version numbers are lousy, precisely because they are not granular enough. That's why the autoconf philosophy frowns so heavily on version checks, and prefers feature checks instead.

We want to know which features are present, not which versions introduced which features. In this case, we want to know about a particular feature (SIGTERM is not broken), which we know exists later than 0.15, but which might also exist as a backport in 0.14 or 0.15. If qemu tells us that information, then upstream libvirt can make the decision correctly regardless of how distros backport the patch. But if qemu does not expose the information, then upstream libvirt must be pessimistic, and you've now forced the distros to do double-duty - they must backport both the qemu fix, and write a distro-specific libvirt patch that alters the version matrix to play with the distro build of qemu.

Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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