• Christoffer Dall's avatar
    KVM: arm/arm64: Avoid timer save/restore in vcpu entry/exit · b103cc3f
    Christoffer Dall authored
    We don't need to save and restore the hardware timer state and examine
    if it generates interrupts on on every entry/exit to the guest.  The
    timer hardware is perfectly capable of telling us when it has expired
    by signaling interrupts.
    
    When taking a vtimer interrupt in the host, we don't want to mess with
    the timer configuration, we just want to forward the physical interrupt
    to the guest as a virtual interrupt.  We can use the split priority drop
    and deactivate feature of the GIC to do this, which leaves an EOI'ed
    interrupt active on the physical distributor, making sure we don't keep
    taking timer interrupts which would prevent the guest from running.  We
    can then forward the physical interrupt to the VM using the HW bit in
    the LR of the GIC, like we do already, which lets the guest directly
    deactivate both the physical and virtual timer simultaneously, allowing
    the timer hardware to exit the VM and generate a new physical interrupt
    when the timer output is again asserted later on.
    
    We do need to capture this state when migrating VCPUs between physical
    CPUs, however, which we use the vcpu put/load functions for, which are
    called through preempt notifiers whenever the thread is scheduled away
    from the CPU or called directly if we return from the ioctl to
    userspace.
    
    One caveat is that we have to save and restore the timer state in both
    kvm_timer_vcpu_[put/load] and kvm_timer_[schedule/unschedule], because
    we can have the following flows:
    
      1. kvm_vcpu_block
      2. kvm_timer_schedule
      3. schedule
      4. kvm_timer_vcpu_put (preempt notifier)
      5. schedule (vcpu thread gets scheduled back)
      6. kvm_timer_vcpu_load (preempt notifier)
      7. kvm_timer_unschedule
    
    And a version where we don't actually call schedule:
    
      1. kvm_vcpu_block
      2. kvm_timer_schedule
      7. kvm_timer_unschedule
    
    Since kvm_timer_[schedule/unschedule] may not be followed by put/load,
    but put/load also may be called independently, we call the timer
    save/restore functions from both paths.  Since they rely on the loaded
    flag to never save/restore when unnecessary, this doesn't cause any
    harm, and we ensure that all invokations of either set of functions work
    as intended.
    
    An added benefit beyond not having to read and write the timer sysregs
    on every entry and exit is that we no longer have to actively write the
    active state to the physical distributor, because we configured the
    irq for the vtimer to only get a priority drop when handling the
    interrupt in the GIC driver (we called irq_set_vcpu_affinity()), and
    the interrupt stays active after firing on the host.
    Reviewed-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
    Signed-off-by: default avatarChristoffer Dall <cdall@linaro.org>
    b103cc3f
arm.c 33.2 KB