Subject: Re: [PATCHv2 2/4] arm64: add guest pvstate support

From: Marc Zyngier <[email-masked]>
Date: Mon, 12 Jul 2021 16:42:37 +0100

On Fri, 09 Jul 2021 05:37:11 +0100,
Sergey Senozhatsky <[email-masked]> wrote:
> 
> PV-vcpu-state is a per-CPU struct, which, for the time being,
> holds boolean `preempted' vCPU state. During the startup,
> given that host supports PV-state, each guest vCPU sends
> a pointer to its per-CPU variable to the host as a payload

What is the expected memory type for this memory region? What is its
life cycle? Where is it allocated from?

> with the SMCCC HV call, so that host can update vCPU state
> when it puts or loads vCPU.
> 
> This has impact on the guest's scheduler:
> 
> [..]
>   wake_up_process()
>    try_to_wake_up()
>     select_task_rq_fair()
>      available_idle_cpu()
>       vcpu_is_preempted()
> 
> Some sched benchmarks data is available on the github page [0].
> 
> [0] https://github.com/sergey-senozhatsky/arm64-vcpu_is_preempted

Please include these results in the cover letter. I tend to reply to
email while offline, and I can't comment on GH.

> Signed-off-by: Sergey Senozhatsky <[email-masked]>
> ---
>  arch/arm64/include/asm/paravirt.h | 19 +++++++
>  arch/arm64/kernel/paravirt.c      | 94 +++++++++++++++++++++++++++++++
>  arch/arm64/kernel/smp.c           |  4 ++
>  3 files changed, 117 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/paravirt.h b/arch/arm64/include/asm/paravirt.h
> index 9aa193e0e8f2..a3f7665dff38 100644
> --- a/arch/arm64/include/asm/paravirt.h
> +++ b/arch/arm64/include/asm/paravirt.h
> @@ -2,6 +2,11 @@
>  #ifndef _ASM_ARM64_PARAVIRT_H
>  #define _ASM_ARM64_PARAVIRT_H
>  
> +struct vcpu_state {

If this is KVM specific (which it most likely is), please name-space
it correctly, and move it to a KVM-specific location.

> +	bool	preempted;
> +	u8	reserved[63];

Why 63? Do you attach any particular meaning to a 64byte structure
(and before you say "cache line size", please look at some of the
cache line sizes we have to deal with...).

This should also be versioned from day-1, one way or another.

> +};
> +
>  #ifdef CONFIG_PARAVIRT
>  #include <linux/static_call_types.h>
>  
> @@ -20,8 +25,22 @@ static inline u64 paravirt_steal_clock(int cpu)
>  
>  int __init pv_time_init(void);
>  
> +bool dummy_vcpu_is_preempted(unsigned int cpu);
> +
> +extern struct static_key pv_vcpu_is_preempted_enabled;
> +DECLARE_STATIC_CALL(pv_vcpu_is_preempted, dummy_vcpu_is_preempted);
> +
> +static inline bool paravirt_vcpu_is_preempted(unsigned int cpu)
> +{
> +	return static_call(pv_vcpu_is_preempted)(cpu);
> +}
> +
> +int __init pv_vcpu_state_init(void);
> +
>  #else
>  
> +#define pv_vcpu_state_init() do {} while (0)
> +
>  #define pv_time_init() do {} while (0)
>  
>  #endif // CONFIG_PARAVIRT
> diff --git a/arch/arm64/kernel/paravirt.c b/arch/arm64/kernel/paravirt.c
> index 75fed4460407..d8fc46795d94 100644
> --- a/arch/arm64/kernel/paravirt.c
> +++ b/arch/arm64/kernel/paravirt.c
> @@ -40,6 +40,11 @@ struct pv_time_stolen_time_region {
>  
>  static DEFINE_PER_CPU(struct pv_time_stolen_time_region, stolen_time_region);
>  
> +static DEFINE_PER_CPU(struct vcpu_state, vcpus_states);

nit: there is only one 'state' structure per CPU, so I'd prefer the
singular form.

> +struct static_key pv_vcpu_is_preempted_enabled;
> +
> +DEFINE_STATIC_CALL(pv_vcpu_is_preempted, dummy_vcpu_is_preempted);
> +
>  static bool steal_acc = true;
>  static int __init parse_no_stealacc(char *arg)
>  {
> @@ -165,3 +170,92 @@ int __init pv_time_init(void)
>  
>  	return 0;
>  }
> +
> +bool dummy_vcpu_is_preempted(unsigned int cpu)

Why does this have to be global?

> +{
> +	return false;
> +}
> +
> +static bool __vcpu_is_preempted(unsigned int cpu)
> +{
> +	struct vcpu_state *st;
> +
> +	st = &per_cpu(vcpus_states, cpu);
> +	return READ_ONCE(st->preempted);
> +}
> +
> +static bool has_pv_vcpu_state(void)
> +{
> +	struct arm_smccc_res res;
> +
> +	/* To detect the presence of PV time support we require SMCCC 1.1+ */
> +	if (arm_smccc_1_1_get_conduit() == SMCCC_CONDUIT_NONE)
> +		return false;
> +
> +	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
> +			     ARM_SMCCC_HV_PV_VCPU_STATE_FEATURES,
> +			     &res);
> +
> +	if (res.a0 != SMCCC_RET_SUCCESS)
> +		return false;
> +	return true;

Please move all this over the the KVM-specific discovery mechanism.

> +}
> +
> +static int __pv_vcpu_state_hook(unsigned int cpu, int event)
> +{
> +	struct arm_smccc_res res;
> +	struct vcpu_state *st;
> +
> +	st = &per_cpu(vcpus_states, cpu);
> +	arm_smccc_1_1_invoke(event, virt_to_phys(st), &res);
> +	if (res.a0 != SMCCC_RET_SUCCESS)
> +		return -EINVAL;
> +	return 0;
> +}
> +
> +static int vcpu_state_init(unsigned int cpu)
> +{
> +	int ret = __pv_vcpu_state_hook(cpu, ARM_SMCCC_HV_PV_VCPU_STATE_INIT);
> +
> +	if (ret)
> +		pr_warn("Unable to ARM_SMCCC_HV_PV_STATE_INIT\n");

pr_warn_once(), please.

> +	return ret;
> +}
> +
> +static int vcpu_state_release(unsigned int cpu)
> +{
> +	int ret = __pv_vcpu_state_hook(cpu, ARM_SMCCC_HV_PV_VCPU_STATE_RELEASE);
> +
> +	if (ret)
> +		pr_warn("Unable to ARM_SMCCC_HV_PV_STATE_RELEASE\n");
> +	return ret;
> +}
> +
> +static int pv_vcpu_state_register_hooks(void)
> +{
> +	int ret;
> +
> +	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
> +				"hypervisor/arm/pvstate:starting",
> +				vcpu_state_init,
> +				vcpu_state_release);
> +	if (ret < 0)
> +		pr_warn("Failed to register CPU hooks\n");
> +	return 0;
> +}
> +
> +int __init pv_vcpu_state_init(void)
> +{
> +	int ret;
> +
> +	if (!has_pv_vcpu_state())
> +		return 0;
> +
> +	ret = pv_vcpu_state_register_hooks();
> +	if (ret)
> +		return ret;
> +
> +	static_call_update(pv_vcpu_is_preempted, __vcpu_is_preempted);
> +	static_key_slow_inc(&pv_vcpu_is_preempted_enabled);
> +	return 0;
> +}
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 6f6ff072acbd..20d42e0f2a99 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -50,6 +50,7 @@
>  #include <asm/tlbflush.h>
>  #include <asm/ptrace.h>
>  #include <asm/virt.h>
> +#include <asm/paravirt.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/ipi.h>
> @@ -756,6 +757,9 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
>  	numa_store_cpu_info(this_cpu);
>  	numa_add_cpu(this_cpu);
>  
> +	/* Init paravirt CPU state */
> +	pv_vcpu_state_init();
> +
>  	/*
>  	 * If UP is mandated by "nosmp" (which implies "maxcpus=0"), don't set
>  	 * secondary CPUs present.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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