Re: [RFC PATCH v13 7/9] arm64/kvm: Add hypercall service for kvm ptp.

[Steven Price <steven.price@arm.com>]

On 19/06/2020 10:30, Jianyong Wu wrote:
> ptp_kvm will get this service through smccc call.
> The service offers wall time and counter cycle of host for guest.
> caller must explicitly determines which cycle of virtual counter or
> physical counter to return if it needs counter cycle.
> 
> Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
> ---
>   arch/arm64/kvm/Kconfig      |  6 +++++
>   arch/arm64/kvm/hypercalls.c | 50 +++++++++++++++++++++++++++++++++++++
>   include/linux/arm-smccc.h   | 30 ++++++++++++++++++++++
>   3 files changed, 86 insertions(+)
> 
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index 13489aff4440..79091f6e5e7a 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -60,6 +60,12 @@ config KVM_ARM_PMU
>   config KVM_INDIRECT_VECTORS
>   	def_bool HARDEN_BRANCH_PREDICTOR || HARDEN_EL2_VECTORS
>   
> +config ARM64_KVM_PTP_HOST
> +	bool "KVM PTP host service for arm64"
> +	default y
> +	help
> +	  virtual kvm ptp clock hypercall service for arm64
> +
>   endif # KVM
>   
>   endif # VIRTUALIZATION
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index db6dce3d0e23..366b0646c360 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -3,6 +3,7 @@
>   
>   #include <linux/arm-smccc.h>
>   #include <linux/kvm_host.h>
> +#include <linux/clocksource_ids.h>
>   
>   #include <asm/kvm_emulate.h>
>   
> @@ -11,6 +12,10 @@
>   
>   int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
>   {
> +#ifdef CONFIG_ARM64_KVM_PTP_HOST
> +	struct system_time_snapshot systime_snapshot;
> +	u64 cycles = 0;
> +#endif
>   	u32 func_id = smccc_get_function(vcpu);
>   	u32 val[4] = {SMCCC_RET_NOT_SUPPORTED};
>   	u32 feature;
> @@ -70,7 +75,52 @@ 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);
> +
> +#ifdef CONFIG_ARM64_KVM_PTP_HOST
> +		val[0] |= BIT(ARM_SMCCC_KVM_FUNC_KVM_PTP);
> +#endif
> +		break;
> +
> +#ifdef CONFIG_ARM64_KVM_PTP_HOST
> +	/*
> +	 * This serves virtual kvm_ptp.
> +	 * Four values will be passed back.
> +	 * reg0 stores high 32-bit host ktime;
> +	 * reg1 stores low 32-bit host ktime;
> +	 * reg2 stores high 32-bit difference of host cycles and cntvoff;
> +	 * reg3 stores low 32-bit difference of host cycles and cntvoff.
> +	 */
> +	case ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID:
> +		/*
> +		 * system time and counter value must captured in the same
> +		 * time to keep consistency and precision.
> +		 */
> +		ktime_get_snapshot(&systime_snapshot);
> +		if (systime_snapshot.cs_id != CSID_ARM_ARCH_COUNTER)
> +			break;
> +		val[0] = upper_32_bits(systime_snapshot.real);
> +		val[1] = lower_32_bits(systime_snapshot.real);
> +		/*
> +		 * which of virtual counter or physical counter being
> +		 * asked for is decided by the first argument of smccc
> +		 * call. If no first argument or invalid argument, zero
> +		 * counter value will return;
> +		 */

It's not actually possible to have "no first argument" - there's no 
argument count, so whatever is in the register during the call with be 
passed. I'd also caution that "first argument" is ambigious: r0 could be 
the 'first' but is also the function number, here you mean r1.

There's also a subtle cast to 32 bits here (feature is u32), which might 
be worth a comment before someone 'optimises' by removing the 'feature' 
variable.

Finally I'm not sure if zero counter value is best - would it not be 
possible for this to be a valid counter value?

> +		feature = smccc_get_arg1(vcpu);
> +		switch (feature) {
> +		case ARM_PTP_VIRT_COUNTER:
> +			cycles = systime_snapshot.cycles -
> +			vcpu_vtimer(vcpu)->cntvoff;

Please indent the continuation line so that it's obvious.

> +			break;
> +		case ARM_PTP_PHY_COUNTER:
> +			cycles = systime_snapshot.cycles;
> +			break;
> +		}
> +		val[2] = upper_32_bits(cycles);
> +		val[3] = lower_32_bits(cycles);
>   		break;
> +#endif
> +
>   	default:
>   		return kvm_psci_call(vcpu);
>   	}
> diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h
> index 86ff30131e7b..e593ec515f82 100644
> --- a/include/linux/arm-smccc.h
> +++ b/include/linux/arm-smccc.h
> @@ -98,6 +98,9 @@
>   
>   /* KVM "vendor specific" services */
>   #define ARM_SMCCC_KVM_FUNC_FEATURES		0
> +#define ARM_SMCCC_KVM_FUNC_KVM_PTP		1
> +#define ARM_SMCCC_KVM_FUNC_KVM_PTP_PHY		2
> +#define ARM_SMCCC_KVM_FUNC_KVM_PTP_VIRT		3
>   #define ARM_SMCCC_KVM_FUNC_FEATURES_2		127
>   #define ARM_SMCCC_KVM_NUM_FUNCS			128
>   
> @@ -107,6 +110,33 @@
>   			   ARM_SMCCC_OWNER_VENDOR_HYP,			\
>   			   ARM_SMCCC_KVM_FUNC_FEATURES)
>   
> +/*
> + * kvm_ptp is a feature used for time sync between vm and host.
> + * kvm_ptp 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)
> +
> +/*
> + * kvm_ptp may get counter cycle from host and should ask for which of
> + * physical counter or virtual counter by using ARM_PTP_PHY_COUNTER and
> + * ARM_PTP_VIRT_COUNTER explicitly.
> + */
> +#define ARM_PTP_PHY_COUNTER						\
> +	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,				\
> +			   ARM_SMCCC_SMC_32,				\
> +			   ARM_SMCCC_OWNER_VENDOR_HYP,			\
> +			   ARM_SMCCC_KVM_FUNC_KVM_PTP_PHY)
> +
> +#define ARM_PTP_VIRT_COUNTER						\
> +	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,				\
> +			   ARM_SMCCC_SMC_32,				\
> +			   ARM_SMCCC_OWNER_VENDOR_HYP,			\
> +			   ARM_SMCCC_KVM_FUNC_KVM_PTP_VIRT)

These two are not SMCCC calls themselves (just parameters to an SMCCC), 
so they really shouldn't be defined using ARM_SMCCC_CALL_VAL (it's just 
confusing and unnecessary). Can we not just pick small integers (e.g. 0 
and 1) for these?

We also need some documentation of these SMCCC calls somewhere which 
would make this sort of review easier. For instance for paravirtualised 
stolen time there is Documentation/virt/kvm/arm/pvtime.rst (which also 
links to the published document from Arm).

Steve

>   #ifndef __ASSEMBLY__
>   
>   #include <linux/linkage.h>
>