Re: [PATCH] Don't call cpu_synchronize_state() in apic_init_reset()

[Avi Kivity <avi@redhat.com>]

On 09/23/2009 06:45 PM, Jan Kiszka wrote:
>> Functions calling each other in the same subsystem can rely on callers
>> calling cpu_synchronize_state().  Across subsystems, that's another
>> matter, exported functions should try not to rely on implementation
>> details of their callers.
>>
>> (You might argue that the apic is not separate subsystem wrt an x86 cpu,
>> and I'm not sure I have a counterargument)
>>
>>      
> I do accept this argument. It's just that my feeling is that we are
> lacking proper review of the required call sites of cpu_sychronize_state
> and rather put it where some regression popped up (and that only in
> qemu-kvm...).
>    

That's life...

> The new rule is: Synchronize the states before accessing registers (or
> in-kernel devices) the first time after a vmexit to user space.

No, the rule is: synchronize state before accessing registers.  Extra 
synchronization is cheap, while missing synchronization is very expensive.

> But,
> e.g., I do not see where we do this on CPU reset.
>    

That's a bug.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.