RE: [Xen-devel] Ehancement to domU suspend/resume

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

OK, seems no comments or against by far. I'm starting coding this 
enhancement now. Of course, comments are still welcomed. :-)

Thanks,
Kevin

>-----Original Message-----
>From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
>[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Tian,
>Kevin
>Sent: 2007年1月17日 17:10
>To: xen-devel@xxxxxxxxxxxxxxxxxxx
>Subject: [Xen-devel] Ehancement to domU suspend/resume
>
>Hi, all,
>       When working on adding PM support to xen, we realized that
>some enhancements are required to suspend/resume domU. Following
>is some background and thoughts, and welcome on comments. :-)
>
>       Currently we use a simple approach (pause/unpause) for domU
>when ready to pull whole platform into a power save state, saying a
>S3. Because pause/unpause is out of domU's knowledge, domU
>observes soft lockup when unpaused after resuming from S3. Also
>this can not handle driver domain case. We tried "xm save/restore"
>or equivalent "xm suspend/resume", it works however overhead is a
>bit high since the whole domU memory is saved to disk and domain
>itself is destroyed after suspend. For S3 support, it's better to have
>quick suspend/resume, cause memory still keeps along the process.
>
>       Basically the change may lie with two aspects:
>               - Lightweight "xm suspend/resume"
>               - Extend suspend support to driver domU
>
>[Lightweight "xm suspend/resume"]
>       It's reasonable for current implementation to save and release
>whole memory of domU after suspended, since it allows more
>memory available to other domains. However for platform level S3,
>this is redundant when box is physically put into a suspend state.
>What we need is just to send a suspend notification into domU, and
>let domU fall into __xen_suspend path. Then domU exits scheduler
>by issuing HYPERVISOR_suspend. Nothing else required after this.
> After resume, domU just continues to run after suspend point.
>
>       Even __xen_suspend path is a bit heavy, and in this case
>resources don't change for domU even after resume. Maybe we
>can benefit from recent checkpoint patch which has appropriate
>change on this path.
>
>       But I'm not familiar with control panel side and hope some guys
>can suggest me. My gut-feeling is to add an option (like -L) to existing
>
>"xm suspend/resume", instead of a new command. Actually the
>possible changes may look like what checkpoint patch does except
>no immediate resume and we need disable memory save logic.
>
>[Driver domU suspend]
>       This has to be added if one device is assigned to a domU and
>we want system still working correctly after resume. When driver
>domU receives suspend request, it should invoke driver suspend
>method of owned physical devices. Before that, one other necessary
>step is to freeze all processes since some may still hold critical
>resource. In this case, we need borrow some Linux PM stuff into
>xen suspend path, something like:
>       Smp_suspend();
>       freeze_processes();
>       device_suspend();
>       device_power_down();
>       xenbus_suspend();
>       ...
>       HYPERVISOR_suspend(virt_to_mfn(xen_start_info));
>       ...
>       xenbus_resume();
>       device_power_up();
>       device_resume();
>       thaw_processes();
>       smp_resume();
>
>       It may be more difficult if we want to support wake-on-LAN when
>that NIC is assigned to domU, which is more tightly related to ACPI.
>So we will simply consider normal device suspend here.
>
>       One interesting question is, why doesn't current __xen_suspend
>freeze running processes? My rough feeling is that virtual device state
>is still kept (like in xenstore, BE, etc.) along with single domU
>suspend/resume, and thus almost all front-end drivers (except TPM)
>have no suspend method. If no driver suspend methods are invoked,
>no need to freeze processes since no contention there. Correct me for
>the real trick. :-)
>
>Thanks,
>Kevin
>
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