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

Zenghui Yu
2019-08-27T12:46:06Z

On 2019/8/23 22:22, Steven Price wrote:
> On 23/08/2019 12:45, Zenghui Yu wrote:
>> 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?
>
> Actually I had run DEBUG_ATOMIC_SLEEP and not seen any issue. But I
> think that's because of the way I've configured the region in my kvmtool
> changes. I'm hitting the path where the memory region is in the linear
> map of the kernel and so no actual remapping is needed and hence
> memremap doesn't sleep (the shared structure is in a reserved region of
> RAM).
>
> But even changing the memory layout of the guest so the call goes into
> ioremap_page_range() (which contains a might_sleep()) I'm not seeing any
> problems.

Emm, I hit this SAC BUG when testing your V1 with the kvmtool changes
you've provided, but forgot to report it at that time. I go back to V1
and get the following call trace:

[ 0.120113] BUG: sleeping function called from invalid context at
mm/slab.h:501
[ 0.120118] in_atomic(): 1, irqs_disabled(): 128, pid: 0, name: swapper/1
[ 0.120122] no locks held by swapper/1/0.
[ 0.120126] irq event stamp: 0
[ 0.120135] hardirqs last enabled at (0): [<0000000000000000>] 0x0
[ 0.120145] hardirqs last disabled at (0): [<ffff200010113b40>]
copy_process+0x870/0x2878
[ 0.120152] softirqs last enabled at (0): [<ffff200010113b40>]
copy_process+0x870/0x2878
[ 0.120157] softirqs last disabled at (0): [<0000000000000000>] 0x0
[ 0.120164] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.3.0-rc6+ #2
[ 0.120168] Hardware name: linux,dummy-virt (DT)
[ 0.120173] Call trace:
[ 0.120179] dump_backtrace+0x0/0x250
[ 0.120184] show_stack+0x24/0x30
[ 0.120192] dump_stack+0x120/0x174
[ 0.120198] ___might_sleep+0x1b0/0x280
[ 0.120203] __might_sleep+0x80/0xf0
[ 0.120209] kmem_cache_alloc_node_trace+0x30c/0x3c8
[ 0.120216] __get_vm_area_node+0x9c/0x208
[ 0.120221] get_vm_area_caller+0x58/0x68
[ 0.120227] __ioremap_caller+0x78/0x120
[ 0.120233] ioremap_cache+0xf0/0x1a8
[ 0.120240] memremap+0x154/0x3b8
[ 0.120245] init_stolen_time_cpu+0x94/0x150
[ 0.120251] cpuhp_invoke_callback+0x12c/0x12f8
[ 0.120257] notify_cpu_starting+0xa0/0xc0
[ 0.120263] secondary_start_kernel+0x1d4/0x328

But things may have changed because we're talking about V3 now...
I will dig it a bit deeper.

> Am I missing something? I have to admit I don't entirely follow the
> early start up - perhaps it's a simple as DEBUG_ATOMIC_SLEEP doesn't
> work this early in boot?

I think it should work.

Thanks,
zenghui