Re: [PATCH 9/9] arm64: Retrieve stolen time as paravirtualized guest

[Steven Price]

On 04/08/2019 10:53, Marc Zyngier wrote:
> On Fri,  2 Aug 2019 15:50:17 +0100
> Steven Price <steven.price@xxxxxxx> wrote:
> 
>> Enable paravirtualization features when running under a hypervisor
>> supporting the PV_TIME_ST hypercall.
>>
>> For each (v)CPU, we ask the hypervisor for the location of a shared
>> page which the hypervisor will use to report stolen time to us. We set
>> pv_time_ops to the stolen time function which simply reads the stolen
>> value from the shared page for a VCPU. We guarantee single-copy
>> atomicity using READ_ONCE which means we can also read the stolen
>> time for another VCPU than the currently running one while it is
>> potentially being updated by the hypervisor.
>>
>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>> ---
>>  arch/arm64/kernel/Makefile |   1 +
>>  arch/arm64/kernel/kvm.c    | 155 +++++++++++++++++++++++++++++++++++++
> 
> nit: Why not using paravirt.c, which clearly states what it does? The
> alternative would be to name it kvm-pv.c.

I can move it to paravirt.c - seems reasonable.

>>  include/linux/cpuhotplug.h |   1 +
>>  3 files changed, 157 insertions(+)
>>  create mode 100644 arch/arm64/kernel/kvm.c
>>
>> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
>> index 478491f07b4f..eb36edf9b930 100644
>> --- a/arch/arm64/kernel/Makefile
>> +++ b/arch/arm64/kernel/Makefile
>> @@ -63,6 +63,7 @@ obj-$(CONFIG_CRASH_CORE)		+= crash_core.o
>>  obj-$(CONFIG_ARM_SDE_INTERFACE)		+= sdei.o
>>  obj-$(CONFIG_ARM64_SSBD)		+= ssbd.o
>>  obj-$(CONFIG_ARM64_PTR_AUTH)		+= pointer_auth.o
>> +obj-$(CONFIG_PARAVIRT)			+= kvm.o
>>  
>>  obj-y					+= vdso/ probes/
>>  obj-$(CONFIG_COMPAT_VDSO)		+= vdso32/
>> diff --git a/arch/arm64/kernel/kvm.c b/arch/arm64/kernel/kvm.c
>> new file mode 100644
>> index 000000000000..245398c79dae
>> --- /dev/null
>> +++ b/arch/arm64/kernel/kvm.c
>> @@ -0,0 +1,155 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +// Copyright (C) 2019 Arm Ltd.
>> +
>> +#define pr_fmt(fmt) "kvmarm-pv: " fmt
>> +
>> +#include <linux/arm-smccc.h>
>> +#include <linux/cpuhotplug.h>
>> +#include <linux/io.h>
>> +#include <linux/printk.h>
>> +#include <linux/psci.h>
>> +#include <linux/reboot.h>
>> +#include <linux/slab.h>
>> +
>> +#include <asm/paravirt.h>
>> +#include <asm/pvclock-abi.h>
>> +#include <asm/smp_plat.h>
>> +
>> +struct kvmarm_stolen_time_region {
>> +	struct pvclock_vcpu_stolen_time_info *kaddr;
>> +};
>> +
>> +static DEFINE_PER_CPU(struct kvmarm_stolen_time_region, stolen_time_region);
>> +
>> +static bool steal_acc = true;
>> +static int __init parse_no_stealacc(char *arg)
>> +{
>> +	steal_acc = false;
>> +	return 0;
>> +}
>> +early_param("no-steal-acc", parse_no_stealacc);
>> +
>> +/* return stolen time in ns by asking the hypervisor */
>> +static u64 kvm_steal_clock(int cpu)
>> +{
>> +	struct kvmarm_stolen_time_region *reg;
>> +
>> +	reg = per_cpu_ptr(&stolen_time_region, cpu);
>> +	if (!reg->kaddr) {
>> +		pr_warn_once("stolen time enabled but not configured for cpu %d\n",
>> +			     cpu);
>> +		return 0;
>> +	}
>> +
>> +	return le64_to_cpu(READ_ONCE(reg->kaddr->stolen_time));
>> +}
>> +
>> +static int disable_stolen_time_current_cpu(void)
>> +{
>> +	struct kvmarm_stolen_time_region *reg;
>> +
>> +	reg = this_cpu_ptr(&stolen_time_region);
>> +	if (!reg->kaddr)
>> +		return 0;
>> +
>> +	memunmap(reg->kaddr);
>> +	memset(reg, 0, sizeof(*reg));
>> +
>> +	return 0;
>> +}
>> +
>> +static int stolen_time_dying_cpu(unsigned int cpu)
>> +{
>> +	return disable_stolen_time_current_cpu();
>> +}
>> +
>> +static int init_stolen_time_cpu(unsigned int cpu)
>> +{
>> +	struct kvmarm_stolen_time_region *reg;
>> +	struct arm_smccc_res res;
>> +
>> +	reg = this_cpu_ptr(&stolen_time_region);
>> +
>> +	if (reg->kaddr)
>> +		return 0;
> 
> Can this actually happen? It'd take two CPU_UP calls from the HP
> notifiers to get in that situation...

Yes, something would have to be very broken for that to happen - I'll
remove this check.

>> +
>> +	arm_smccc_1_1_invoke(ARM_SMCCC_HV_PV_TIME_ST, &res);
>> +
>> +	if ((long)res.a0 < 0)
>> +		return -EINVAL;
>> +
>> +	reg->kaddr = memremap(res.a0,
>> +			sizeof(struct pvclock_vcpu_stolen_time_info),
>> +			MEMREMAP_WB);
>> +
>> +	if (reg->kaddr == NULL) {
>> +		pr_warn("Failed to map stolen time data structure\n");
>> +		return -EINVAL;
> 
> -ENOMEM is the expected return code.

Ok

>> +	}
>> +
>> +	if (le32_to_cpu(reg->kaddr->revision) != 0 ||
>> +			le32_to_cpu(reg->kaddr->attributes) != 0) {
>> +		pr_warn("Unexpected revision or attributes in stolen time data\n");
>> +		return -ENXIO;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +static int kvm_arm_init_stolen_time(void)
>> +{
>> +	int ret;
>> +
>> +	ret = cpuhp_setup_state(CPUHP_AP_ARM_KVMPV_STARTING,
>> +				"hypervisor/kvmarm/pv:starting",
>> +				init_stolen_time_cpu, stolen_time_dying_cpu);
>> +	if (ret < 0)
>> +		return ret;
>> +	return 0;
>> +}
>> +
>> +static bool has_kvm_steal_clock(void)
>> +{
>> +	struct arm_smccc_res res;
>> +
>> +	/* To detect the presence of PV time support we require SMCCC 1.1+ */
>> +	if (psci_ops.smccc_version < SMCCC_VERSION_1_1)
>> +		return false;
>> +
>> +	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
>> +			     ARM_SMCCC_HV_PV_FEATURES, &res);
>> +
>> +	if (res.a0 != SMCCC_RET_SUCCESS)
>> +		return false;
>> +
>> +	arm_smccc_1_1_invoke(ARM_SMCCC_HV_PV_FEATURES,
>> +			     ARM_SMCCC_HV_PV_TIME_ST, &res);
>> +
>> +	if (res.a0 != SMCCC_RET_SUCCESS)
>> +		return false;
>> +
>> +	return true;
>> +}
>> +
>> +static int __init kvm_guest_init(void)
>> +{
>> +	int ret = 0;
>> +
>> +	if (!has_kvm_steal_clock())
>> +		return 0;
>> +
>> +	ret = kvm_arm_init_stolen_time();
>> +	if (ret)
>> +		return ret;
>> +
>> +	pv_ops.time.steal_clock = kvm_steal_clock;
>> +
>> +	static_key_slow_inc(&paravirt_steal_enabled);
>> +	if (steal_acc)
>> +		static_key_slow_inc(&paravirt_steal_rq_enabled);
>> +
>> +	pr_info("using stolen time PV\n");
>> +
>> +	return 0;
>> +}
>> +early_initcall(kvm_guest_init);
> 
> Is there any reason why we wouldn't directly call into this rather than
> using an initcall?

I'm not sure where the direct call would go - any pointers?

Thanks,

Steve

>> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
>> index 068793a619ca..89d75edb5750 100644
>> --- a/include/linux/cpuhotplug.h
>> +++ b/include/linux/cpuhotplug.h
>> @@ -136,6 +136,7 @@ enum cpuhp_state {
>>  	/* Must be the last timer callback */
>>  	CPUHP_AP_DUMMY_TIMER_STARTING,
>>  	CPUHP_AP_ARM_XEN_STARTING,
>> +	CPUHP_AP_ARM_KVMPV_STARTING,
>>  	CPUHP_AP_ARM_CORESIGHT_STARTING,
>>  	CPUHP_AP_ARM64_ISNDEP_STARTING,
>>  	CPUHP_AP_SMPCFD_DYING,
> 
> 
> Thanks,
> 
> 	M.
>