Re: [PATCH v2 3/4] x86/kvm/hyper-v: direct mode for synthetic timers

[Roman Kagan]

On Mon, Nov 26, 2018 at 04:47:31PM +0100, Vitaly Kuznetsov wrote:
> Turns out Hyper-V on KVM (as of 2016) will only use synthetic timers
> if direct mode is available. With direct mode we notify the guest by
> asserting APIC irq instead of sending a SynIC message.
> 
> The implementation uses existing vec_bitmap for letting lapic code
> know that we're interested in the particular IRQ's EOI request. We assume
> that the same APIC irq won't be used by the guest for both direct mode
> stimer and as sint source (especially with AutoEOI semantics). It is
> unclear how things should be handled if that's not true.
> 
> Direct mode is also somewhat less expensive; in my testing
> stimer_send_msg() takes not less than 1500 cpu cycles and
> stimer_notify_direct() can usually be done in 300-400. WS2016 without
> Hyper-V, however, always sticks to non-direct version.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
> - Changes since v1: avoid open-coding stimer_mark_pending() in
>   kvm_hv_synic_send_eoi() [Paolo Bonzini]
> ---
>  arch/x86/kvm/hyperv.c    | 67 +++++++++++++++++++++++++++++++++++-----
>  arch/x86/kvm/trace.h     | 10 +++---
>  arch/x86/kvm/x86.c       |  1 +
>  include/uapi/linux/kvm.h |  1 +
>  4 files changed, 67 insertions(+), 12 deletions(-)

Reviewed-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>