Re: [PATCH v1 2/3] x86/xen/time: setup vcpu 0 time info page

[Joao Martins]

On 01/25/2017 07:26 PM, Boris Ostrovsky wrote:
> On 01/25/2017 12:33 PM, Joao Martins wrote:
>> In order to support pvclock vdso on xen we need to setup the time
>> info page for vcpu 0 and register the page with Xen using the
>> VCPUOP_register_vcpu_time_memory_area hypercall. This hypercall
>> will also forcefully update the pvti which will set some of the
>> necessary flags for vdso. Afterwards we check if it supports the
>> PVCLOCK_TSC_STABLE_BIT flag which is mandatory for having
>> vdso/vsyscall support. And if so, it will set the cpu 0 pvti that
>> will be later on used when mapping the vdso image.
>>
>> The xen headers are also updated to include the new hypercall for
>> registering the secondary vcpu_time_info struct.
>>
>> Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
>> ---
>> Changes since RFC:
>>  (Comments from Boris and David)
>>  * Remove Kconfig option
>>  * Use get_zeroed_page/free/page
>>  * Remove the hypercall availability check
>>  * Unregister pvti with arg.addr.v = NULL if stable bit isn't supported.
>>  (New)
>>  * Set secondary copy on restore such that it works on migration.
>>  * Drop global xen_clock variable and stash it locally on
>>  xen_setup_vsyscall_time_info.
>>  * WARN_ON(ret) if we fail to unregister the pvti.
>> ---
>>  arch/x86/xen/enlighten.c     |  2 ++
>>  arch/x86/xen/time.c          | 51 ++++++++++++++++++++++++++++++++++++++++++++
>>  arch/x86/xen/xen-ops.h       |  1 +
>>  include/xen/interface/vcpu.h | 28 ++++++++++++++++++++++++
>>  4 files changed, 82 insertions(+)
>>
>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>> index 51ef952..15d271d 100644
>> --- a/arch/x86/xen/enlighten.c
>> +++ b/arch/x86/xen/enlighten.c
>> @@ -270,6 +270,8 @@ void xen_vcpu_restore(void)
> 
> This is called for PV only. What about HVM?
The call is missing from xen_hvm_post_suspend(...), I will add it.

> 
>>  		    HYPERVISOR_vcpu_op(VCPUOP_up, xen_vcpu_nr(cpu), NULL))
>>  			BUG();
>>  	}
>> +
>> +	xen_setup_vsyscall_time_info(0);
> 
> Do we need to tear down time memory area on VCPU suspend?
I also missed that; otherwise I am leaking a page.

I could also rework this patch such that the initially allocated xen_clock page
is reused and simply register/unregister in save/restore paths. This would
probably mean adding one extra helper to register the vcpu_time info and perhaps
make xen_setup_vsyscall_time_info a bit simpler.

>>  }
>>  
>>  static void __init xen_banner(void)
>> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
>> index 1e69956..e90f703 100644
>> --- a/arch/x86/xen/time.c
>> +++ b/arch/x86/xen/time.c
>> @@ -367,6 +367,56 @@ static const struct pv_time_ops xen_time_ops __initconst = {
>>  	.steal_clock = xen_steal_clock,
>>  };
>>  
>> +int xen_setup_vsyscall_time_info(int cpu)
>> +{
>> +	struct pvclock_vsyscall_time_info *xen_clock;
>> +	struct vcpu_register_time_memory_area t;
>> +	struct pvclock_vcpu_time_info *pvti;
>> +	unsigned long addr;
>> +	u8 flags;
>> +	int ret;
>> +
>> +	addr = get_zeroed_page(GFP_KERNEL);
>> +	if (!addr)
>> +		return -ENOMEM;
>> +
>> +	xen_clock = (struct pvclock_vsyscall_time_info *) addr;
>> +	memset(xen_clock, 0, PAGE_SIZE);
> 
> You don't really need addr 
The reason I had this was to avoid to save one cast to unsigned long (on
free_page paths). But maybe it's not worth it and looking at the rest of the
x86/xen code, this doesn't seem to be the case. I will remove it.

> and there is no reason to memset the page to
> zero, given that you got it with get_zeroed_page().
Yeap, Fixed.

> 
>> +
>> +	t.addr.v = &xen_clock->pvti;
>> +
>> +	ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area,
>> +				 cpu, &t);
>> +
>> +	if (ret) {
>> +		pr_debug("xen: cannot register vcpu_time_info err %d\n", ret);
> 
> pr_warn() would be more appropriate I think. You also have blank line
> before 'if'.
Fixed.

>> +		free_page(addr);
>> +		return ret;
>> +	}
>> +
>> +	pvti = &xen_clock->pvti;
>> +	flags = pvti->flags;
> 
> I don't think you need these, given that you only reference flags once
> below.
> 
Indeed, fixed as well.

>> +
>> +	if (!(flags & PVCLOCK_TSC_STABLE_BIT)) {
>> +		t.addr.v = NULL;
>> +		ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area,
>> +					 cpu, &t);
>> +		if (!ret)
>> +			free_page(addr);
>> +
>> +		WARN_ON(ret);
> 
> Did someone ask for WARN_ON()? (from your changelog it looks like it
> could have been either David or me). I think pr_warn() is sufficient,
> just like above.
Looking again at previous comments no, this WARN_ON wasn't asked. I can changed
it to the similar above as you are suggesting.

> 
>> +		pr_debug("xen: VCLOCK_PVCLOCK not supported\n");
> 
> pr_notice()
Fixed it too.

> 
> -boris
> 
>> +		return -ENOTSUPP;
>> +	}
>> +
>> +	pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
>> +	pvclock_set_pvti_cpu0_va(xen_clock);
>> +
>> +	xen_clocksource.archdata.vclock_mode = VCLOCK_PVCLOCK;
>> +
>> +	return 0;
>> +}
>> +
>>  static void __init xen_time_init(void)
>>  {
>>  	int cpu = smp_processor_id();
>> @@ -393,6 +443,7 @@ static void __init xen_time_init(void)
>>  	setup_force_cpu_cap(X86_FEATURE_TSC);
>>  
>>  	xen_setup_runstate_info(cpu);
>> +	xen_setup_vsyscall_time_info(cpu);
>>  	xen_setup_timer(cpu);
>>  	xen_setup_cpu_clockevents();
>>  
>> diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
>> index ac0a2b0..4036d15 100644
>> --- a/arch/x86/xen/xen-ops.h
>> +++ b/arch/x86/xen/xen-ops.h
>> @@ -66,6 +66,7 @@ void __init xen_vmalloc_p2m_tree(void);
>>  void xen_init_irq_ops(void);
>>  void xen_setup_timer(int cpu);
>>  void xen_setup_runstate_info(int cpu);
>> +int xen_setup_vsyscall_time_info(int cpu);
>>  void xen_teardown_timer(int cpu);
>>  u64 xen_clocksource_read(void);
>>  void xen_setup_cpu_clockevents(void);
>> diff --git a/include/xen/interface/vcpu.h b/include/xen/interface/vcpu.h
>> index 98188c8..8da788c 100644
>> --- a/include/xen/interface/vcpu.h
>> +++ b/include/xen/interface/vcpu.h
>> @@ -178,4 +178,32 @@ DEFINE_GUEST_HANDLE_STRUCT(vcpu_register_vcpu_info);
>>  
>>  /* Send an NMI to the specified VCPU. @extra_arg == NULL. */
>>  #define VCPUOP_send_nmi             11
>> +
>> +/*
>> + * Register a memory location to get a secondary copy of the vcpu time
>> + * parameters.  The master copy still exists as part of the vcpu shared
>> + * memory area, and this secondary copy is updated whenever the master copy
>> + * is updated (and using the same versioning scheme for synchronisation).
>> + *
>> + * The intent is that this copy may be mapped (RO) into userspace so
>> + * that usermode can compute system time using the time info and the
>> + * tsc.  Usermode will see an array of vcpu_time_info structures, one
>> + * for each vcpu, and choose the right one by an existing mechanism
>> + * which allows it to get the current vcpu number (such as via a
>> + * segment limit).  It can then apply the normal algorithm to compute
>> + * system time from the tsc.
>> + *
>> + * @extra_arg == pointer to vcpu_register_time_info_memory_area structure.
>> + */
>> +#define VCPUOP_register_vcpu_time_memory_area   13
>> +DEFINE_GUEST_HANDLE_STRUCT(vcpu_time_info_t);
>> +struct vcpu_register_time_memory_area {
>> +	union {
>> +		GUEST_HANDLE(vcpu_time_info_t) h;
>> +		struct pvclock_vcpu_time_info *v;
>> +		uint64_t p;
>> +	} addr;
>> +};
>> +DEFINE_GUEST_HANDLE_STRUCT(vcpu_register_time_memory_area_t);
>> +
>>  #endif /* __XEN_PUBLIC_VCPU_H__ */
> 
>