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

Steven Price
2019-10-21T12:15:01Z

On 19/10/2019 21:28, Marc Zyngier wrote:
> On Fri, 11 Oct 2019 13:59:30 +0100,
> Steven Price <steven.price@arm.com> 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>
>> ---
>> .../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 <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)
>
> This isn't KVM specific.

Good point - these changes have grown from an initial KVM-only
implementation, but this is no longer KVM specific in any way. So I'll
rename the KVM part away.

>> +{
>> + 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.

Will do

>> +
>> +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...

I suspect we'd have a whole lot of other problems with 64 bit IPAs, but
fair enough; we know the one error code that can be returned.

>> +
>> + 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...

Good point.

>> + 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).

pv_time_init() seems like a reasonable name, it rolls off the tongue
better than den0057 :)

Thanks,

Steve

>> +{
>> + 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.
>