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    SubjectRe: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation
    On Friday, 13 July 2007 18:48, Jeremy Maitin-Shepard wrote:
    > "Rafael J. Wysocki" <> writes:
    > [snip]
    > > Okay, I have thought it through and I think that, as an initial step, we can do
    > > something like this:
    > > - preload the image-saving kernel before hibernation
    > > - in the hibernation code path replace device_suspend() with the shutting down
    > > of
    > > all devices without unregistering them (not very nice, but should be
    > > sufficient
    > > for a while)
    > It seems that the effect of what is done by the current hibernate
    > implementations is to shutdown all of the devices, but according to
    > kernel data structures, have it look like the devices were merely
    > suspended (i.e. device_suspend). Then in the resume path, the "restore
    > image" kernel also calls device_suspend just before jumping to the
    > hibernated kernel, so all of the devices the "restore image" kernel knew
    > about are in the state device_suspend expects them to be in, except that
    > they were actually suspended by a different kernel, so they might not be
    > in quite the right state. There is also the issue that the "restore
    > image" kernel might not know about all of the devices; for instance, if
    > USB support is modular, and, as is likely to be the case, the user
    > didn't load the USB modules in the "restore image" kernel from an initrd
    > or something, then the USB devices will actually be powered off, rather
    > than "suspended". Despite these apparent discrepancies, it seems that
    > for many devices (I'm not sure USB devices are included, though),
    > device_restore happens to do the right thing so that the device is
    > placed back in the state it needs to be so that the driver can begin
    > talking to it as it did before, and the device is recognized as the same
    > device as was there before (since otherwise mounted filesystems backed
    > by block devices that came back as a different device would cause great
    > havoc).
    > Since I recall there being issues with USB devices being recognized as
    > the same devices post-hibernate-resume, without looking at the code I'm
    > inclined to believe that the USB drivers still don't end up resuming
    > from hibernate correctly.
    > Note that I am describing what is done currently, not what is planned to
    > be done (i.e. change device_suspend to quiesce and device_resume to
    > unquiesce). It seems that ironically, despite everyone believing that
    > device_suspend/device_resume is incorrect for hibernate, many of the
    > things that those functions do (like saving the PCI configuration,
    > perhaps, and then restoring it later, or re-initializing the device) are
    > actually necessary, especially for modular drivers that won't be loaded
    > by the "restore image" kernel.
    > What needs to be done is for the devices to be shut down (or possibly
    > just quiesced for a select few, but we won't worry about that
    > complication until later; in the case of the current implementations,
    > they should all be quiesced rather than shut down), but whatever
    > information that will be needed later to reinitialize the device
    > (ideally the reinitialization should be able to handle the device either
    > being in a quiesced state or completely off) and recognize it as the
    > same device must be saved. This probably means they cannot be
    > "unregistered", as otherwise there would be nothing with which to
    > associate the saved information.
    > The resume path needs to use the saved state to reinitialize the device
    > and recognize it as the same device. It seems that the existing
    > reprobing code may not be sufficient for this. Note that exactly the
    > same thing must be done on resume for both the current hibernate
    > implementations and the kexec approach. It seems that properly
    > restoring the devices should be relatively easy for the devices that
    > already get this correct, like IDE devices and basic PCI devices (and
    > SATA and SCSI devices as well perhaps?), and possibly harder for
    > Firewire or USB devices.

    The problem of handling devices during hibernation has been discussed for many
    times on linux-pm and we have reached certain agreement. I wouldn't like to
    repeat all of the arguments here. For details, please refer, for example, to
    this thread:

    > > - when we've called device_power_down() and save_processor_state(), jump to
    > > the image-saving kernel and let it run
    > > - make the image-saving kernel set up everything, save the image without
    > > starting any user space (we may use the existing image-saving code for this
    > > purpose, with some modifications) and power off the system (or make it enter
    > > S4)
    > I suppose this has the advantage of not requiring that a
    > kernel-to-userspace interface be created for this purpose.

    Yes, and for now we're avoiding the problems with starting the new user space
    from a special place.

    > > - use the existing restoration code to load the image and jump to the
    > > hibernated kernel
    > This would again avoid the need for a separate userspace-kernelspace
    > interface for the purpose, so I agree it could be a useful thing to do
    > initially.
    > > - in the restore code patch replace device_resume() with the reprobing of all
    > > devices.
    > See my comments above.

    And please see my reply. :-)


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