linux-kernel - Re: [PATCH v15 6/9] arm64/kvm: Add hypercall service for kvm ptp.

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Message-ID: <d409aa1cb7cfcbf4351e6c5fc34d9c7e@kernel.org>
Date:   Mon, 23 Nov 2020 10:44:12 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Jianyong Wu <jianyong.wu@....com>
Cc:     netdev@...r.kernel.org, yangbo.lu@....com, john.stultz@...aro.org,
        tglx@...utronix.de, pbonzini@...hat.com,
        sean.j.christopherson@...el.com, richardcochran@...il.com,
        Mark.Rutland@....com, will@...nel.org, suzuki.poulose@....com,
        Andre.Przywara@....com, steven.price@....com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        Steve.Capper@....com, justin.he@....com, nd@....com
Subject: Re: [PATCH v15 6/9] arm64/kvm: Add hypercall service for kvm ptp.

On 2020-11-11 06:22, Jianyong Wu wrote:
> ptp_kvm will get this service through SMCC call.
> The service offers wall time and cycle count of host to guest.
> The caller must specify whether they want the host cycle count
> or the difference between host cycle count and cntvoff.
> 
> Signed-off-by: Jianyong Wu <jianyong.wu@....com>
> ---
>  arch/arm64/kvm/hypercalls.c | 61 +++++++++++++++++++++++++++++++++++++
>  include/linux/arm-smccc.h   | 17 +++++++++++
>  2 files changed, 78 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index b9d8607083eb..f7d189563f3d 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -9,6 +9,51 @@
>  #include <kvm/arm_hypercalls.h>
>  #include <kvm/arm_psci.h>
> 
> +static void kvm_ptp_get_time(struct kvm_vcpu *vcpu, u64 *val)
> +{
> +	struct system_time_snapshot systime_snapshot;
> +	u64 cycles = ~0UL;
> +	u32 feature;
> +
> +	/*
> +	 * system time and counter value must captured in the same
> +	 * time to keep consistency and precision.
> +	 */
> +	ktime_get_snapshot(&systime_snapshot);
> +
> +	// binding ptp_kvm clocksource to arm_arch_counter
> +	if (systime_snapshot.cs_id != CSID_ARM_ARCH_COUNTER)
> +		return;
> +
> +	val[0] = upper_32_bits(systime_snapshot.real);
> +	val[1] = lower_32_bits(systime_snapshot.real);

What is the endianness of these values? I can't see it defined
anywhere, and this is likely not to work if guest and hypervisor
don't align.

> +
> +	/*
> +	 * which of virtual counter or physical counter being
> +	 * asked for is decided by the r1 value of SMCCC
> +	 * call. If no invalid r1 value offered, default cycle
> +	 * value(-1) will be returned.
> +	 * Note: keep in mind that feature is u32 and smccc_get_arg1
> +	 * will return u64, so need auto cast here.
> +	 */
> +	feature = smccc_get_arg1(vcpu);
> +	switch (feature) {
> +	case ARM_PTP_VIRT_COUNTER:
> +		cycles = systime_snapshot.cycles - vcpu_read_sys_reg(vcpu, 
> CNTVOFF_EL2);
> +		break;
> +	case ARM_PTP_PHY_COUNTER:
> +		cycles = systime_snapshot.cycles;
> +		break;
> +	case ARM_PTP_NONE_COUNTER:

What is this "NONE" counter?

> +		break;
> +	default:
> +		val[0] = SMCCC_RET_NOT_SUPPORTED;
> +		break;
> +	}
> +	val[2] = upper_32_bits(cycles);
> +	val[3] = lower_32_bits(cycles);

Same problem as above.

> +}
> +
>  int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
>  {
>  	u32 func_id = smccc_get_function(vcpu);
> @@ -79,6 +124,22 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
>  		break;
>  	case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
>  		val[0] = BIT(ARM_SMCCC_KVM_FUNC_FEATURES);
> +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_KVM_PTP);
> +		break;
> +	/*
> +	 * This serves virtual kvm_ptp.
> +	 * Four values will be passed back.
> +	 * reg0 stores high 32-bits of host ktime;
> +	 * reg1 stores low 32-bits of host ktime;
> +	 * For ARM_PTP_VIRT_COUNTER:
> +	 * reg2 stores high 32-bits of difference of host cycles and cntvoff;
> +	 * reg3 stores low 32-bits of difference of host cycles and cntvoff.
> +	 * For ARM_PTP_PHY_COUNTER:
> +	 * reg2 stores the high 32-bits of host cycles;
> +	 * reg3 stores the low 32-bits of host cycles.
> +	 */
> +	case ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID:
> +		kvm_ptp_get_time(vcpu, val);
>  		break;
>  	default:
>  		return kvm_psci_call(vcpu);
> diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h
> index d75408141137..a03c5dd409d3 100644
> --- a/include/linux/arm-smccc.h
> +++ b/include/linux/arm-smccc.h
> @@ -103,6 +103,7 @@
> 
>  /* KVM "vendor specific" services */
>  #define ARM_SMCCC_KVM_FUNC_FEATURES		0
> +#define ARM_SMCCC_KVM_FUNC_KVM_PTP		1

I think having KVM once in the name is enough.

>  #define ARM_SMCCC_KVM_FUNC_FEATURES_2		127
>  #define ARM_SMCCC_KVM_NUM_FUNCS			128
> 
> @@ -114,6 +115,22 @@
> 
>  #define SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED	1
> 
> +/*
> + * ptp_kvm is a feature used for time sync between vm and host.
> + * ptp_kvm module in guest kernel will get service from host using
> + * this hypercall ID.
> + */
> +#define ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID				\
> +	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,				\
> +			   ARM_SMCCC_SMC_32,				\
> +			   ARM_SMCCC_OWNER_VENDOR_HYP,			\
> +			   ARM_SMCCC_KVM_FUNC_KVM_PTP)
> +
> +/* ptp_kvm counter type ID */
> +#define ARM_PTP_VIRT_COUNTER			0
> +#define ARM_PTP_PHY_COUNTER			1
> +#define ARM_PTP_NONE_COUNTER			2

The architecture definitely doesn't have this last counter.

> +
>  /* Paravirtualised time calls (defined by ARM DEN0057A) */
>  #define ARM_SMCCC_HV_PV_TIME_FEATURES				\
>  	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,			\

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...