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Date: Tue, 02 Feb 2021 13:32:25 +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 v16 6/9] arm64/kvm: Add hypercall service for kvm ptp.
On 2020-12-09 06:09, 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 | 59 +++++++++++++++++++++++++++++++++++++
> include/linux/arm-smccc.h | 16 ++++++++++
> 2 files changed, 75 insertions(+)
>
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index b9d8607083eb..9a4834502388 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -9,6 +9,49 @@
> #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);
> +
> + /*
> + * 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.
This feels like a useless comment. On the other hand, you don't
document why we don't need to worry about -1 being used as the
top 32bits of the real time.
> + */
> + feature = smccc_get_arg1(vcpu);
> + switch (feature) {
> + case ARM_PTP_VIRT_COUNTER:
This #define should at least contain "KVM", because this isn't
something that is architectural.
> + cycles = systime_snapshot.cycles - vcpu_read_sys_reg(vcpu,
> CNTVOFF_EL2);
> + break;
> + case ARM_PTP_PHY_COUNTER:
s/PHY/PHYS/
> + cycles = systime_snapshot.cycles;
> + break;
> + default:
> + val[0] = SMCCC_RET_NOT_SUPPORTED;
> + break;
> + }
> + val[2] = upper_32_bits(cycles);
> + val[3] = lower_32_bits(cycles);
The control flow is really odd. You start by writing values to
the return array, and then have to overwrite it if you fail.
I came up with the following instead (including comments):
@@ -16,38 +16,44 @@ static void kvm_ptp_get_time(struct kvm_vcpu *vcpu,
u64 *val)
u32 feature;
/*
- * system time and counter value must captured in the same
+ * system time and counter value must captured at the same
* time to keep consistency and precision.
*/
ktime_get_snapshot(&systime_snapshot);
- // binding ptp_kvm clocksource to arm_arch_counter
+ /*
+ * This is only valid if the current clocksource is the
+ * architected counter, as this is the only one the guest
+ * can see.
+ */
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);
-
/*
- * 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.
+ * The guest selects one of the two reference counters
+ * (virtual or physical) with the first argument of the SMCCC
+ * call. In case the identifier is not supported, error out.
*/
feature = smccc_get_arg1(vcpu);
switch (feature) {
- case ARM_PTP_VIRT_COUNTER:
+ case KVM_PTP_VIRT_COUNTER:
cycles = systime_snapshot.cycles - vcpu_read_sys_reg(vcpu,
CNTVOFF_EL2);
break;
- case ARM_PTP_PHY_COUNTER:
+ case KVM_PTP_PHYS_COUNTER:
cycles = systime_snapshot.cycles;
break;
default:
- val[0] = SMCCC_RET_NOT_SUPPORTED;
- break;
+ return;
}
+
+ /*
+ * This relies on the top bit of val[0] never being set for
+ * valid values of system time, because that is *really* far
+ * in the future (about 292 years from 1970, and at that stage
+ * nobody will give a damn about it).
+ */
+ val[0] = upper_32_bits(systime_snapshot.real);
+ val[1] = lower_32_bits(systime_snapshot.real);
val[2] = upper_32_bits(cycles);
val[3] = lower_32_bits(cycles);
}
> +}
> +
> int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
> {
> u32 func_id = smccc_get_function(vcpu);
> @@ -79,6 +122,22 @@ 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_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.
> + */
You have a patch with documentation in this series, and I don't think
this adds much.
> + 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..7924069f8f0a 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_PTP 1
> #define ARM_SMCCC_KVM_FUNC_FEATURES_2 127
> #define ARM_SMCCC_KVM_NUM_FUNCS 128
>
> @@ -114,6 +115,21 @@
>
> #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_PTP)
> +
> +/* ptp_kvm counter type ID */
> +#define ARM_PTP_VIRT_COUNTER 0
> +#define ARM_PTP_PHY_COUNTER 1
+#define KVM_PTP_VIRT_COUNTER 0
+#define KVM_PTP_PHYS_COUNTER 1
> +
> /* 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...
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