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

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Date:   Mon, 10 Dec 2018 12:06:41 +0000
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>
Subject: Re: [PATCH v2 3/4] x86/kvm/hyper-v: direct mode for synthetic timers

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@...hat.com>
> ---
> - 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(-)
> 
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index eaec15c738df..9533133be566 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -38,6 +38,9 @@
>  
>  #define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64)
>  
> +static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,
> +				bool vcpu_kick);
> +
>  static inline u64 synic_read_sint(struct kvm_vcpu_hv_synic *synic, int sint)
>  {
>  	return atomic64_read(&synic->sint[sint]);
> @@ -53,8 +56,21 @@ static inline int synic_get_sint_vector(u64 sint_value)
>  static bool synic_has_vector_connected(struct kvm_vcpu_hv_synic *synic,
>  				      int vector)
>  {
> +	struct kvm_vcpu *vcpu = synic_to_vcpu(synic);
> +	struct kvm_vcpu_hv *hv_vcpu = vcpu_to_hv_vcpu(vcpu);
> +	struct kvm_vcpu_hv_stimer *stimer;
>  	int i;
>  
> +	for (i = 0; i < ARRAY_SIZE(hv_vcpu->stimer); i++) {
> +		stimer = &hv_vcpu->stimer[i];
> +		if (stimer->config.enable && stimer->config.direct_mode &&
> +		    stimer->config.apic_vector == vector)
> +			return true;
> +	}
> +
> +	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
> +		return false;
> +
>  	for (i = 0; i < ARRAY_SIZE(synic->sint); i++) {
>  		if (synic_get_sint_vector(synic_read_sint(synic, i)) == vector)
>  			return true;
> @@ -80,14 +96,14 @@ static bool synic_has_vector_auto_eoi(struct kvm_vcpu_hv_synic *synic,
>  static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
>  				int vector)
>  {
> -	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
> -		return;
> -
>  	if (synic_has_vector_connected(synic, vector))
>  		__set_bit(vector, synic->vec_bitmap);
>  	else
>  		__clear_bit(vector, synic->vec_bitmap);
>  
> +	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
> +		return;
> +

Just noticed that the patch seems to assume that "direct" timers are
allowed to use any vectors including 0-15.  I guess this is incorrect,
and instead stimer_set_config should error out on direct mode with a
vector less than HV_SYNIC_FIRST_VALID_VECTOR.

Roman.