[Qemu-devel] Re: [PATCH v2 14/21] qemu-kvm: Rework VCPU state writeback API

[Avi Kivity]

On 02/03/2010 10:53 AM, 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_ASYNC_STATE (during runtime, other VCPUs continue to run)
>    

Wouldn't that be SYNC_STATE (state that is modified by the current vcpu 
only)?

> - 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. It does
> not touch qemu-kvm's special hooks for mpstate, vcpu_events, or tsc
> loading. They will be cleaned up by individual patches.
>
>    

I'm uneasy about this.  What are the rules for putting 
cpu_synchronize_state() now?


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