Subject: Re: [PATCH v6 10/10] arm64: Retrieve stolen time as paravirtualized guest

From: Marc Zyngier <[email-masked]>
Date: Sat, 19 Oct 2019 21:28:39 +0100

On Fri, 11 Oct 2019 13:59:30 +0100,
Steven Price <[email-masked]> 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 <[email-masked]>
> ---
>  .../admin-guide/kernel-parameters.txt         |   6 +-
>  arch/arm64/include/asm/paravirt.h             |   9 +-
>  arch/arm64/kernel/paravirt.c                  | 148 ++++++++++++++++++
>  arch/arm64/kernel/time.c                      |   3 +
>  include/linux/cpuhotplug.h                    |   1 +
>  5 files changed, 163 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c7ac2f3ac99f..346b1c7a4afb 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3083,9 +3083,9 @@
>  			[X86,PV_OPS] Disable paravirtualized VMware scheduler
>  			clock and use the default one.
>  
> -	no-steal-acc	[X86,KVM] Disable paravirtualized steal time accounting.
> -			steal time is computed, but won't influence scheduler
> -			behaviour
> +	no-steal-acc	[X86,KVM,ARM64] Disable paravirtualized steal time
> +			accounting. steal time is computed, but won't
> +			influence scheduler behaviour
>  
>  	nolapic		[X86-32,APIC] Do not enable or use the local APIC.
>  
> diff --git a/arch/arm64/include/asm/paravirt.h b/arch/arm64/include/asm/paravirt.h
> index 799d9dd6f7cc..125c26c42902 100644
> --- a/arch/arm64/include/asm/paravirt.h
> +++ b/arch/arm64/include/asm/paravirt.h
> @@ -21,6 +21,13 @@ static inline u64 paravirt_steal_clock(int cpu)
>  {
>  	return pv_ops.time.steal_clock(cpu);
>  }
> -#endif
> +
> +int __init kvm_guest_init(void);
> +
> +#else
> +
> +#define kvm_guest_init()
> +
> +#endif // CONFIG_PARAVIRT
>  
>  #endif
> diff --git a/arch/arm64/kernel/paravirt.c b/arch/arm64/kernel/paravirt.c
> index 4cfed91fe256..de73dbec238c 100644
> --- a/arch/arm64/kernel/paravirt.c
> +++ b/arch/arm64/kernel/paravirt.c
> @@ -6,13 +6,161 @@
>   * Author: Stefano Stabellini <[email-masked]>
>   */
>  
> +#define pr_fmt(fmt) "kvmarm-pv: " fmt
> +
> +#include <linux/arm-smccc.h>
> +#include <linux/cpuhotplug.h>
>  #include <linux/export.h>
> +#include <linux/io.h>
>  #include <linux/jump_label.h>
> +#include <linux/printk.h>
> +#include <linux/psci.h>
> +#include <linux/reboot.h>
> +#include <linux/slab.h>
>  #include <linux/types.h>
> +
>  #include <asm/paravirt.h>
> +#include <asm/pvclock-abi.h>
> +#include <asm/smp_plat.h>
>  
>  struct static_key paravirt_steal_enabled;
>  struct static_key paravirt_steal_rq_enabled;
>  
>  struct paravirt_patch_template pv_ops;
>  EXPORT_SYMBOL_GPL(pv_ops);
> +
> +struct kvmarm_stolen_time_region {
> +	struct pvclock_vcpu_stolen_time *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)

This isn't KVM specific.

> +{
> +	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();
> +}

You can merge these two functions, as there is no other caller.

> +
> +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);
> +
> +	arm_smccc_1_1_invoke(ARM_SMCCC_HV_PV_TIME_ST, &res);
> +
> +	if ((long)res.a0 < 0)
> +		return -EINVAL;

I'd rather you check an actual error code, just in case the memory map
starts growing to a point where we have 64bit (I)PAs...

> +
> +	reg->kaddr = memremap(res.a0,
> +			      sizeof(struct pvclock_vcpu_stolen_time),
> +			      MEMREMAP_WB);
> +
> +	if (!reg->kaddr) {
> +		pr_warn("Failed to map stolen time data structure\n");
> +		return -ENOMEM;
> +	}
> +
> +	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");

WARN_ONCE instead? You probably don't want to scream for each and
every CPU that boots...

> +		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)

This is not KVM specific either.

> +{
> +	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_TIME_FEATURES, &res);
> +
> +	if (res.a0 != SMCCC_RET_SUCCESS)
> +		return false;
> +
> +	arm_smccc_1_1_invoke(ARM_SMCCC_HV_PV_TIME_FEATURES,
> +			     ARM_SMCCC_HV_PV_TIME_ST, &res);
> +
> +	if (res.a0 != SMCCC_RET_SUCCESS)
> +		return false;
> +
> +	return true;

	return (res.a0 == SMCCC_RET_SUCCESS);

> +}
> +
> +int __init kvm_guest_init(void)

How about something like pv_time_init() instead? In the guest, this is
no way KVM specific, and I still hope for this to work on things like
Xen/HyperV/VMware (yeah, I'm foolishly optimistic). All the references
to KVM should go, and be replaced by something more generic (after
all, you're only implementing the spec, so feel free to call it
den0057_* if you really want).

> +{
> +	int ret;
> +
> +	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;
> +}
> diff --git a/arch/arm64/kernel/time.c b/arch/arm64/kernel/time.c
> index 0b2946414dc9..a52aea14c6ec 100644
> --- a/arch/arm64/kernel/time.c
> +++ b/arch/arm64/kernel/time.c
> @@ -30,6 +30,7 @@
>  
>  #include <asm/thread_info.h>
>  #include <asm/stacktrace.h>
> +#include <asm/paravirt.h>
>  
>  unsigned long profile_pc(struct pt_regs *regs)
>  {
> @@ -65,4 +66,6 @@ void __init time_init(void)
>  
>  	/* Calibrate the delay loop directly */
>  	lpj_fine = arch_timer_rate / HZ;
> +
> +	kvm_guest_init();
>  }
> 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,
> -- 
> 2.20.1
> 
> 

Thanks,

	M.

-- 
Jazz is not dead, it just smells funny.

_______________________________________________
linux-arm-kernel mailing list
[email-masked]
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel