• Nicholas Piggin's avatar
    powerpc/64s: Reimplement book3s idle code in C · 10d91611
    Nicholas Piggin authored
    Reimplement Book3S idle code in C, moving POWER7/8/9 implementation
    speific HV idle code to the powernv platform code.
    
    Book3S assembly stubs are kept in common code and used only to save
    the stack frame and non-volatile GPRs before executing architected
    idle instructions, and restoring the stack and reloading GPRs then
    returning to C after waking from idle.
    
    The complex logic dealing with threads and subcores, locking, SPRs,
    HMIs, timebase resync, etc., is all done in C which makes it more
    maintainable.
    
    This is not a strict translation to C code, there are some
    significant differences:
    
    - Idle wakeup no longer uses the ->cpu_restore call to reinit SPRs,
      but saves and restores them itself.
    
    - The optimisation where EC=ESL=0 idle modes did not have to save GPRs
      or change MSR is restored, because it's now simple to do. ESL=1
      sleeps that do not lose GPRs can use this optimization too.
    
    - KVM secondary entry and cede is now more of a call/return style
      rather than branchy. nap_state_lost is not required because KVM
      always returns via NVGPR restoring path.
    
    - KVM secondary wakeup from offline sequence is moved entirely into
      the offline wakeup, which avoids a hwsync in the normal idle wakeup
      path.
    
    Performance measured with context switch ping-pong on different
    threads or cores, is possibly improved a small amount, 1-3% depending
    on stop state and core vs thread test for shallow states. Deep states
    it's in the noise compared with other latencies.
    
    KVM improvements:
    
    - Idle sleepers now always return to caller rather than branch out
      to KVM first.
    
    - This allows optimisations like very fast return to caller when no
      state has been lost.
    
    - KVM no longer requires nap_state_lost because it controls NVGPR
      save/restore itself on the way in and out.
    
    - The heavy idle wakeup KVM request check can be moved out of the
      normal host idle code and into the not-performance-critical offline
      code.
    
    - KVM nap code now returns from where it is called, which makes the
      flow a bit easier to follow.
    Reviewed-by: default avatarGautham R. Shenoy <ego@linux.vnet.ibm.com>
    Signed-off-by: default avatarNicholas Piggin <npiggin@gmail.com>
    [mpe: Squash the KVM changes in]
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    10d91611
paca.h 8.59 KB