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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
