Re: [Qemu-devel] [PATCH 00/21] Kemari for KVM 0.2

[Stefan Hajnoczi <stefanha@gmail.com>]

On Sat, Nov 27, 2010 at 4:29 AM, Yoshiaki Tamura
<tamura.yoshiaki@lab.ntt.co.jp> wrote:
> 2010/11/27 Blue Swirl <blauwirbel@gmail.com>:
>> On Thu, Nov 25, 2010 at 6:06 AM, Yoshiaki Tamura
>> <tamura.yoshiaki@lab.ntt.co.jp> wrote:
>>> Hi,
>>>
>>> This patch series is a revised version of Kemari for KVM, which
>>> applied comments for the previous post and KVM Forum 2010.  The
>>> current code is based on qemu.git
>>> f711df67d611e4762966a249742a5f7499e19f99.
>>>
>>> For general information about Kemari, I've made a wiki page at
>>> qemu.org.
>>>
>>> http://wiki.qemu.org/Features/FaultTolerance
>>>
>>> The changes from v0.1.1 -> v0.2 are:
>>>
>>> - Introduce a queue in event-tap to make VM sync live.
>>> - Change transaction receiver to a state machine for async receiving.
>>> - Replace net/block layer functions with event-tap proxy functions.
>>> - Remove dirty bitmap optimization for now.
>>> - convert DPRINTF() in ft_trans_file to trace functions.
>>> - convert fprintf() in ft_trans_file to error_report().
>>> - improved error handling in ft_trans_file.
>>> - add a tmp pointer to qemu_del_vm_change_state_handler.
>>>
>>> The changes from v0.1 -> v0.1.1 are:
>>>
>>> - events are tapped in net/block layer instead of device emulation layer.
>>> - Introduce a new option for -incoming to accept FT transaction.
>>> - Removed writev() support to QEMUFile and FdMigrationState for now.  I would
>>>  post this work in a different series.
>>> - Modified virtio-blk save/load handler to send inuse variable to
>>>  correctly replay.
>>> - Removed configure --enable-ft-mode.
>>> - Removed unnecessary check for qemu_realloc().
>>>
>>> The first 6 patches modify several functions of qemu to prepare
>>> introducing Kemari specific components.
>>>
>>> The next 6 patches are the components of Kemari.  They introduce
>>> event-tap and the FT transaction protocol file based on buffered file.
>>> The design document of FT transaction protocol can be found at,
>>> http://wiki.qemu.org/images/b/b1/Kemari_sender_receiver_0.5a.pdf
>>>
>>> Then the following 4 patches modifies dma-helpers, virtio-blk
>>> virtio-net and e1000 to replace net/block layer functions with
>>> event-tap proxy functions.  Please note that if Kemari is off,
>>> event-tap will just passthrough, and there is most no intrusion to
>>> exisiting functions including normal live migration.
>>
>> Would it be possible to make the changes only in the block/net layer,
>> so that the devices are not modified at all? That is, the proxy
>> function would always replaces the unproxied version.
>
> I understand the benefit of your suggestion.  However it seems a bit
> tricky.  It's because event-tap uses functions of emulators and net,
> but block.c is also linked for utilities like qemu-img that doesn't
> need emulators or net.  In the previous version, I added function
> pointers to get around.
>
> http://lists.nongnu.org/archive/html/qemu-devel/2010-05/msg02378.html
>
> I wasn't confident of this approach and discussed it at KVM Forum, and
> decided to give a try to replace emulator functions with proxies.
> Suggestions are welcomed of course.
>
>> Somehow I find some similarities to instrumentation patches. Perhaps
>> the instrumentation framework could be used (maybe with some changes)
>> for Kemari as well? That could be beneficial to both.
>
> Yes.  I had the same idea but I'm not sure how tracing works.  I think
> Stefan Hajnoczi knows it better.
>
> Stefan, is it possible to call arbitrary functions from the trace
> points?

Yes, if you add code to ./tracetool.  I'm not sure I see the
connection between Kemari and tracing though.

One question I have about Kemari is whether it adds new constraints to
the QEMU codebase?  Fault tolerance seems like a cross-cutting concern
- everyone writing device emulation or core QEMU code may need to be
aware of new constraints.  For example, "you are not allowed to
release I/O operations to the outside world directly, instead you need
to go through Kemari code which makes I/O transactional and
communicates with the passive host".  You have converted e1000,
virtio-net, and virtio-blk.  How do we make sure new devices that are
merged into qemu.git don't break Kemari?  How do we go about
supporting the existing hw/* devices?

Stefan