[Qemu-devel] Re: [PATCH 2/4] KVM: Rework VCPU state writeback API

[Jan Kiszka <jan.kiszka@siemens.com>]

Marcelo Tosatti wrote:
> On Tue, Mar 02, 2010 at 09:00:04AM +0100, Jan Kiszka wrote:
>> Marcelo Tosatti wrote:
>>> On Mon, Mar 01, 2010 at 07:10:30PM +0100, Jan Kiszka wrote:
>>>> This grand cleanup drops all reset and vmsave/load related
>>>> synchronization points in favor of four(!) generic hooks:
>>>>
>>>> - cpu_synchronize_all_states in qemu_savevm_state_complete
>>>>   (initial sync from kernel before vmsave)
>>>> - cpu_synchronize_all_post_init in qemu_loadvm_state
>>>>   (writeback after vmload)
>>>> - cpu_synchronize_all_post_init in main after machine init
>>>> - cpu_synchronize_all_post_reset in qemu_system_reset
>>>>   (writeback after system reset)
>>>>
>>>> These writeback points + the existing one of VCPU exec after
>>>> cpu_synchronize_state map on three levels of writeback:
>>>>
>>>> - KVM_PUT_RUNTIME_STATE (during runtime, other VCPUs continue to run)
>>>> - KVM_PUT_RESET_STATE   (on synchronous system reset, all VCPUs stopped)
>>>> - KVM_PUT_FULL_STATE    (on init or vmload, all VCPUs stopped as well)
>>>>
>>>> This level is passed to the arch-specific VCPU state writing function
>>>> that will decide which concrete substates need to be written. That way,
>>>> no writer of load, save or reset functions that interact with in-kernel
>>>> KVM states will ever have to worry about synchronization again. That
>>>> also means that a lot of reasons for races, segfaults and deadlocks are
>>>> eliminated.
>>>>
>>>> cpu_synchronize_state remains untouched, just as Anthony suggested. We
>>>> continue to need it before reading or writing of VCPU states that are
>>>> also tracked by in-kernel KVM subsystems.
>>>>
>>>> Consequently, this patch removes many cpu_synchronize_state calls that
>>>> are now redundant, just like remaining explicit register syncs.
>>>>
>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>> Jan,
>>>
>>> This patch breaks system reset of WinXP.32 install (more easily
>>> reproducible without iothread enabled).
>>>
>>> Screenshot attached.
>>>
>> Strange - no issues with qemu-kvm? Any special command line switch? /me
>> goes scrounging for some installation XP32 CD in the meantime...
> 
> No issues with qemu-kvm. Could not spot anything obvious.
> 

And, of course, my WinXP installation did not trigger any reset issue,
even in non-iothreaded mode. :(

Jan

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