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(). > reality. That's why not synchronizing properly is "expensive" (or broken > IOW). > > Jan > -- 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
