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    SubjectRe: [PATCHv2 2/4] arm64: add guest pvstate support
    Hi, Just few nits, patch itself LGTM:

    On Fri, Jul 9, 2021 at 12:37 AM Sergey Senozhatsky
    <senozhatsky@chromium.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
    > 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
    >
    > Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.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 {
    > + bool preempted;
    > + u8 reserved[63];
    > +};
    > +
    > #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;.

    pv_vcpu_is_preempted_enabled static_key is not used in any patch.
    Maybe it is stale?

    > +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);
    > +struct static_key pv_vcpu_is_preempted_enabled;
    > +
    > +DEFINE_STATIC_CALL(pv_vcpu_is_preempted, dummy_vcpu_is_preempted);

    Could we use DEFINE_STATIC_CALL_NULL and get rid of the dummy
    function? I believe that makes the function trampoline as return
    instruction, till it is updated.

    > +
    > 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)
    > +{
    > + 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);

    I guess you could just do:
    {
    return READ_ONCE(per_cpu(vcpus_states, cpu).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;
    > +}
    > +
    > +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");
    > + 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);

    I think this static key inc is also stale.

    thanks,

    -Joel

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