Re: [PATCH 0/2] Fix hangup after creating checkpoint on Xen.

On Tue, 2011-02-08 at 11:46 -0500, Alan Stern wrote:

On Tue, 8 Feb 2011, Ian Campbell wrote:

quoted

The problem is that currently we have:

        dpm_suspend_start(PMSG_SUSPEND);
        
                dpm_suspend_noirq(PMSG_SUSPEND);
                        
                        sysdev_suspend(PMSG_SUSPEND);
                        /* suspend hypercall */
                        sysdev_resume();
                
                dpm_resume_noirq(PMSG_RESUME);
        
        dpm_resume_end(PMSG_RESUME);

However the suspend hypercall can return a value indicating that the
suspend didn't actually happen (e.g. was cancelled). This is used e.g.
when checkpointing guests, because in that case you want the original
guest to continue. When the suspend didn't happen the drivers need to
recover differently from if it did.

That is odd, and it is quite different from the intended design of the 
PM core.  Drivers are supposed to put their devices into a known 
suspended state; then afterwards they put the devices back into an 
operational state.  What happens while the devices are in the suspended 
state isn't supposed to matter -- the system transition can fail, but 
devices get treated exactly the same way as if it succeeded.

Why do your drivers need to recover differently based on the success of 
the hypercall?

checkpointing isn't really my area but AIUI you don't want to do a full
device teardown and reconnect like you would with a proper suspend
because of the time that takes which prevents you from doing continuous
rolling checkpoints at granularity which people want to implement
various disaster recovery schemes.

Hopefully one of the Xen checkpointing folks will chime in and explain
why this is not possible to achieve at the individual driver level (or,
even better, with a patch which does it that way ;-)).

Ian.

-- 
Ian Campbell
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