Gleb Natapov wrote: > 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. Yes, I see now. Still, it's not a good reference for other archs (did anyone already checked the situation on ppc?). Will file a patch. Jan -- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux -- 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
