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

[Eric Blake <eblake@redhat.com>]

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
>> corrupt
>> data on a patched 0.14 or 0.15 qemu.
>
> Can you be a lot more specific about what bug you mean?
>

https://bugzilla.redhat.com/show_bug.cgi?id=739895

>> 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   eblake@redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org