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