Re: [PATCH v3 10/10] arm64: Retrieve stolen time as paravirtualized guest

Zenghui Yu
2019-08-23T11:48:57Z

Hi Steven,

On 2019/8/21 23:36, Steven Price 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@arm.com>
> ---
> arch/arm64/include/asm/paravirt.h | 9 +-
> arch/arm64/kernel/paravirt.c | 148 ++++++++++++++++++++++++++++++
> arch/arm64/kernel/time.c | 3 +
> include/linux/cpuhotplug.h | 1 +
> 4 files changed, 160 insertions(+), 1 deletion(-)
>
> 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..ea8dbbbd3293 100644
> --- a/arch/arm64/kernel/paravirt.c
> +++ b/arch/arm64/kernel/paravirt.c
> @@ -6,13 +6,161 @@
> * Author: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> */
>
> +#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)
> +{
> + 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);
> +
> + 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),
> + MEMREMAP_WB);

cpuhp callbacks can be invoked in atomic context (see:
secondary_start_kernel ->
notify_cpu_starting ->
invoke callbacks),
but memremap might sleep...

Try to run a DEBUG_ATOMIC_SLEEP enabled PV guest, I guess we will be
greeted by the Sleep-in-Atomic-Context BUG. We need an alternative
here?

> +
> + 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");
> + 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;
> +}
> +
> +int __init kvm_guest_init(void)
> +{
> + int ret = 0;

And this look like a redundant initialization?

Thanks,
zenghui

> +
> + 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,
>