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Date: Mon, 12 Jul 2021 16:42:37 +0100
From: Marc Zyngier <maz@...nel.org>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Will Deacon <will@...nel.org>,
Suleiman Souhlal <suleiman@...gle.com>,
Joel Fernandes <joelaf@...gle.com>,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCHv2 2/4] arm64: add guest pvstate support
On Fri, 09 Jul 2021 05:37:11 +0100,
Sergey Senozhatsky <senozhatsky@...omium.org> 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 <senozhatsky@...omium.org>
> ---
> 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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