On Thu, Sep 24, 2009 at 10:59:46AM +0200, Jan Kiszka wrote:
> Gleb Natapov wrote:
> > On Thu, Sep 24, 2009 at 10:15:15AM +0200, Jan Kiszka wrote:
> >> Gleb Natapov wrote:
> >>> 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.
> >> No, that's not enough if cpu_reset() first fiddles with some registers
> >> that may later on be overwritten on cpu_synchronize_state() with the old
> >> in-kernel state. At least in theory, haven't checked yet what happens in
> > Can't happen. Call chain is apic_reset() -> cpu_reset() and apic_reset()
> > calls cpu_synchronize_state() before calling cpu_reset().
>
> And system_reset?
>
And system_reset calls apic_reset() if cpu has apic, cpu_reset()
otherwise. That is why I said that the bug is only for cpus without
apic.
--
Gleb.
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