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

[Gleb Natapov <gleb@redhat.com>]

On Thu, Sep 24, 2009 at 10:53:59AM +0300, Avi Kivity wrote:
> 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.
> 
So should we stick cpu_synchronize_state() before each register
accesses? I think it is reasonable to omit it if all callers do it
already.

> >But,
> >e.g., I do not see where we do this on CPU reset.
> 
> That's a bug.
> 
Only if kvm support cpus without apic. Otherwise CPU is reset by 
apic_reset() and cpu_synchronize_state() is called there.

--
			Gleb.