Re: [patch 2/4] KVM: x86: add KVM_HC_CLOCK_OFFSET hypercall

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On 25/01/2017 13:55, Marcelo Tosatti wrote:
> On Wed, Jan 25, 2017 at 01:30:49PM +0100, Paolo Bonzini wrote:
>>
>>
>> On 24/01/2017 18:09, Marcelo Tosatti wrote:
>>> Add a hypercall to retrieve the host realtime clock
>>> and the TSC value used to calculate that clock read.
>>>
>>> Used to implement clock synchronization between
>>> host and guest.
>>>
>>> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
>>>
>>> ---
>>>  Documentation/virtual/kvm/hypercalls.txt |   33 ++++++++++++++++++++++++
>>>  arch/x86/include/uapi/asm/kvm_para.h     |    9 ++++++
>>>  arch/x86/kvm/x86.c                       |   41 +++++++++++++++++++++++++++++++
>>>  include/uapi/linux/kvm_para.h            |    3 ++
>>>  4 files changed, 86 insertions(+)
>>>
>>> v2: improve documentation (Radim)
>>>     change hypercall name to KVM_HC_CLOCK_PAIRING (Radim)
>>>     increase padding size
>>>
>>> Index: kvm-ptpdriver/arch/x86/include/uapi/asm/kvm_para.h
>>> ===================================================================
>>> --- kvm-ptpdriver.orig/arch/x86/include/uapi/asm/kvm_para.h	2017-01-13 16:43:11.947240575 -0200
>>> +++ kvm-ptpdriver/arch/x86/include/uapi/asm/kvm_para.h	2017-01-13 16:43:25.038258187 -0200
>>> @@ -50,6 +50,15 @@
>>>  	__u32 pad[11];
>>>  };
>>>  
>>> +#define KVM_CLOCK_PAIRING_WALLCLOCK 0
>>> +struct kvm_clock_offset {
>>
>> The struct still has the old name.
>>
>>> +	__s64 sec;
>>> +	__s64 nsec;
>>> +	__u64 tsc;
>>> +	__u32 flags;
>>> +	__u32 pad[9];
>>> +};
> 
> That was on purpose: "to pair" is the operation. 
> 
> "struct kvm_clock_pairing" makes no sense to me 
> (because pairing is an "action").
> 
> (if you'd prefer that name, let me know).

A "pairing" is a union of two things, in this case a sec/nsec value and
a TSC value.  (I had to look it up in the dictionary though :)).

>>>  #define KVM_STEAL_ALIGNMENT_BITS 5
>>>  #define KVM_STEAL_VALID_BITS ((-1ULL << (KVM_STEAL_ALIGNMENT_BITS + 1)))
>>>  #define KVM_STEAL_RESERVED_MASK (((1 << KVM_STEAL_ALIGNMENT_BITS) - 1 ) << 1)
>>> Index: kvm-ptpdriver/Documentation/virtual/kvm/hypercalls.txt
>>> ===================================================================
>>> --- kvm-ptpdriver.orig/Documentation/virtual/kvm/hypercalls.txt	2017-01-13 16:43:11.947240575 -0200
>>> +++ kvm-ptpdriver/Documentation/virtual/kvm/hypercalls.txt	2017-01-13 16:43:25.038258187 -0200
>>> @@ -81,3 +81,36 @@
>>>  same guest can wakeup the sleeping vcpu by issuing KVM_HC_KICK_CPU hypercall,
>>>  specifying APIC ID (a1) of the vcpu to be woken up. An additional argument (a0)
>>>  is used in the hypercall for future use.
>>> +
>>> +
>>> +6. KVM_HC_CLOCK_PAIRING
>>> +------------------------
>>> +Architecture: x86
>>> +Status: active
>>> +Purpose: Hypercall used to synchronize host and guest clocks.
>>> +Usage:
>>> +
>>> +a0: guest physical address where host copies
>>> +"struct kvm_clock_offset" structure.
>>> +
>>> +a1: clock_type, ATM only KVM_CLOCK_PAIRING_WALLCLOCK (0)
>>> +is supported (hosts CLOCK_REALTIME clock).
>>> +
>>> +		struct kvm_clock_offset {
>>
>> Name of the struct not adjusted.
>>> +			__s64 sec;
>>> +			__s64 nsec;
>>> +			__u64 tsc;
>>> +			__u32 flags;
>>> +			__u32 pad[9];
>>
>> I'd just leave one element of padding and put the struct on the stack, 
>> since we have no reason to think we'll need a lot of padding.
> 
> Well you don't know: we could implement it in case of HPET clocksource
> hosts.

But then the guest wouldn't have a TSC value to use, would it?

Also, how would it use more padding?

Paolo

>>> +		};
>>> +
>>> +       Where:
>>> +               * sec: seconds from clock_type clock.
>>> +               * nsec: nanoseconds from clock_type clock.
>>> +               * tsc: TSC value used to calculate sec/nsec pair
>>> +                 (this hypercall only works when host uses TSC clocksource).
>>
>> This is already mentioned below, and it's not clear that tsc is a guest 
>> TSC value.
> 
> Ok fixed.
> 
>>> +               * flags: flags, unused (0) at the moment.
>>> +
>>> +Returns KVM_EOPNOTSUPP if the host does not use TSC clocksource,
>>> +or if clock type is different than KVM_CLOCK_PAIRING_WALLCLOCK.
>>> +
>>> Index: kvm-ptpdriver/arch/x86/kvm/x86.c
>>> ===================================================================
>>> --- kvm-ptpdriver.orig/arch/x86/kvm/x86.c	2017-01-13 16:43:20.898252596 -0200
>>> +++ kvm-ptpdriver/arch/x86/kvm/x86.c	2017-01-13 16:43:25.041258191 -0200
>>> @@ -6119,6 +6119,44 @@
>>>  }
>>>  EXPORT_SYMBOL_GPL(kvm_emulate_halt);
>>>  
>>> +static int kvm_pv_clock_pairing(struct kvm_vcpu *vcpu, gpa_t paddr,
>>> +			       unsigned long clock_type)
>>> +{
>>> +	struct kvm_clock_offset *clock_offset;
>>> +	struct timespec ts;
>>> +	cycle_t cycle;
>>> +	int ret;
>>> +
>>> +	if (clock_type != KVM_CLOCK_PAIRING_WALLCLOCK)
>>> +		return -KVM_EOPNOTSUPP;
>>> +
>>> +	if (pvclock_gtod_data.clock.vclock_mode != VCLOCK_TSC)
>>> +		return -KVM_EOPNOTSUPP;
>>
>> Already checked in kvm_get_walltime_and_clockread.
> 
> Fixed.
> 
>>> +	clock_offset = kzalloc(sizeof(struct kvm_clock_offset), GFP_KERNEL);
>>> +	if (clock_offset == NULL)
>>> +		return -KVM_ENOMEM;
>>> +
>>> +	if (kvm_get_walltime_and_clockread(&ts, &cycle) == false) {
>>> +		kfree(clock_offset);
>>> +		return -KVM_EOPNOTSUPP;
>>> +	}
>>> +	clock_offset->sec = ts.tv_sec;
>>> +	clock_offset->nsec = ts.tv_nsec;
>>> +	clock_offset->tsc = kvm_read_l1_tsc(vcpu, cycle);
>>> +	clock_offset->flags = 0;
>>> +
>>> +	ret = 0;
>>> +	if (kvm_write_guest(vcpu->kvm, paddr, clock_offset,
>>> +			      sizeof(struct kvm_clock_offset)))
>>> +		ret = -KVM_EFAULT;
>>> +
>>> +	kfree(clock_offset);
>>> +
>>> +	return ret;
>>> +}
>>> +
>>>  /*
>>>   * kvm_pv_kick_cpu_op:  Kick a vcpu.
>>>   *
>>> @@ -6183,6 +6221,9 @@
>>>  		kvm_pv_kick_cpu_op(vcpu->kvm, a0, a1);
>>>  		ret = 0;
>>>  		break;
>>> +	case KVM_HC_CLOCK_PAIRING:
>>> +		ret = kvm_pv_clock_pairing(vcpu, a0, a1);
>>> +		break;
>>>  	default:
>>>  		ret = -KVM_ENOSYS;
>>>  		break;
>>> Index: kvm-ptpdriver/include/uapi/linux/kvm_para.h
>>> ===================================================================
>>> --- kvm-ptpdriver.orig/include/uapi/linux/kvm_para.h	2017-01-13 16:43:11.947240575 -0200
>>> +++ kvm-ptpdriver/include/uapi/linux/kvm_para.h	2017-01-13 16:43:25.042258192 -0200
>>> @@ -14,6 +14,8 @@
>>>  #define KVM_EFAULT		EFAULT
>>>  #define KVM_E2BIG		E2BIG
>>>  #define KVM_EPERM		EPERM
>>> +#define KVM_EOPNOTSUPP		EOPNOTSUPP
>>> +#define KVM_ENOMEM		ENOMEM
>>
>> This file could potentially be used across multiple architectures (it's 
>> not in arch/x86/include/uapi/asm), but EOPNOTSUPP is not the same on 
>> all systems.  Better use "95" which is the value it has on x86.
> 
> Fixed.
> 
>> All this gives:
>>
>> diff --git a/Documentation/virtual/kvm/hypercalls.txt b/Documentation/virtual/kvm/hypercalls.txt
>> index 55eb3e1ca494..cd35de84999f 100644
>> --- a/Documentation/virtual/kvm/hypercalls.txt
>> +++ b/Documentation/virtual/kvm/hypercalls.txt
>> @@ -94,9 +94,9 @@ a0: guest physical address where host copies
>>  "struct kvm_clock_offset" structure.
>>  
>>  a1: clock_type, ATM only KVM_CLOCK_PAIRING_WALLCLOCK (0)
>> -is supported (hosts CLOCK_REALTIME clock).
>> +is supported (corresponding to the host's CLOCK_REALTIME clock).
>>  
>> -		struct kvm_clock_offset {
>> +		struct kvm_clock_pairing {
>>  			__s64 sec;
>>  			__s64 nsec;
>>  			__u64 tsc;
>> @@ -107,10 +107,12 @@ is supported (hosts CLOCK_REALTIME clock).
>>         Where:
>>                 * sec: seconds from clock_type clock.
>>                 * nsec: nanoseconds from clock_type clock.
>> -               * tsc: TSC value used to calculate sec/nsec pair
>> -                 (this hypercall only works when host uses TSC clocksource).
>> +               * tsc: guest TSC value used to calculate sec/nsec pair
>>                 * flags: flags, unused (0) at the moment.
>>  
>> +The hypercall lets a guest compute a precise timestamp across
>> +host and guest.  The guest can use the returned TSC value to
>> +compute the CLOCK_REALTIME for its clock, at the same instant.
>> +
>>  Returns KVM_EOPNOTSUPP if the host does not use TSC clocksource,
>>  or if clock type is different than KVM_CLOCK_PAIRING_WALLCLOCK.
>> -
>> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
>> index ce7b411e28bc..3641d8b7dba9 100644
>> --- a/arch/x86/include/uapi/asm/kvm_para.h
>> +++ b/arch/x86/include/uapi/asm/kvm_para.h
>> @@ -51,12 +51,12 @@ struct kvm_steal_time {
>>  };
>>  
>>  #define KVM_CLOCK_PAIRING_WALLCLOCK 0
>> -struct kvm_clock_offset {
>> +struct kvm_clock_pairing {
>>  	__s64 sec;
>>  	__s64 nsec;
>>  	__u64 tsc;
>>  	__u32 flags;
>> -	__u32 pad[9];
>> +	__u32 pad[1];
>>  };
>>  
>>  #define KVM_STEAL_ALIGNMENT_BITS 5
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 4ecc3145ea4b..09e5d31dac98 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -6149,9 +6149,9 @@ int kvm_emulate_halt(struct kvm_vcpu *vcpu)
>>  EXPORT_SYMBOL_GPL(kvm_emulate_halt);
>>  
>>  static int kvm_pv_clock_pairing(struct kvm_vcpu *vcpu, gpa_t paddr,
>> -			       unsigned long clock_type)
>> +			        unsigned long clock_type)
>>  {
>> -	struct kvm_clock_offset *clock_offset;
>> +	struct kvm_clock_pairing clock_pairing;
>>  	struct timespec ts;
>>  	cycle_t cycle;
>>  	int ret;
>> @@ -6159,30 +6159,19 @@ static int kvm_pv_clock_pairing(struct kvm_vcpu *vcpu, gpa_t paddr,
>>  	if (clock_type != KVM_CLOCK_PAIRING_WALLCLOCK)
>>  		return -KVM_EOPNOTSUPP;
>>  
>> -	if (pvclock_gtod_data.clock.vclock_mode != VCLOCK_TSC)
>> -		return -KVM_EOPNOTSUPP;
>> -
>> -	clock_offset = kzalloc(sizeof(struct kvm_clock_offset), GFP_KERNEL);
>> -	if (clock_offset == NULL)
>> -		return -KVM_ENOMEM;
>> -
>> -	if (kvm_get_walltime_and_clockread(&ts, &cycle) == false) {
>> -		kfree(clock_offset);
>> +	if (kvm_get_walltime_and_clockread(&ts, &cycle) == false)
>>  		return -KVM_EOPNOTSUPP;
>> -	}
>>  
>> -	clock_offset->sec = ts.tv_sec;
>> -	clock_offset->nsec = ts.tv_nsec;
>> -	clock_offset->tsc = kvm_read_l1_tsc(vcpu, cycle);
>> -	clock_offset->flags = 0;
>> +	clock_pairing.sec = ts.tv_sec;
>> +	clock_pairing.nsec = ts.tv_nsec;
>> +	clock_pairing.tsc = kvm_read_l1_tsc(vcpu, cycle);
>> +	clock_pairing.flags = 0;
>>  
>>  	ret = 0;
>> -	if (kvm_write_guest(vcpu->kvm, paddr, clock_offset,
>> -			      sizeof(struct kvm_clock_offset)))
>> +	if (kvm_write_guest(vcpu->kvm, paddr, &clock_pairing,
>> +			    sizeof(struct kvm_clock_pairing)))
>>  		ret = -KVM_EFAULT;
>>  
>> -	kfree(clock_offset);
>> -
>>  	return ret;
>>  }
>>  
>> diff --git a/include/uapi/linux/kvm_para.h b/include/uapi/linux/kvm_para.h
>> index 68da60e7f519..c72128527377 100644
>> --- a/include/uapi/linux/kvm_para.h
>> +++ b/include/uapi/linux/kvm_para.h
>> @@ -14,7 +14,7 @@
>>  #define KVM_EFAULT		EFAULT
>>  #define KVM_E2BIG		E2BIG
>>  #define KVM_EPERM		EPERM
>> -#define KVM_EOPNOTSUPP		EOPNOTSUPP
>> +#define KVM_EOPNOTSUPP		95
>>  #define KVM_ENOMEM		ENOMEM
>>  
>>  #define KVM_HC_VAPIC_POLL_IRQ		1
>>
>> Okay?
> 
> Just not the padding please, the rest is fine.
> 
>>
>> Paolo
>>
>>>  #define KVM_HC_VAPIC_POLL_IRQ		1
>>>  #define KVM_HC_MMU_OP			2
>>> @@ -23,6 +25,7 @@
>>>  #define KVM_HC_MIPS_GET_CLOCK_FREQ	6
>>>  #define KVM_HC_MIPS_EXIT_VM		7
>>>  #define KVM_HC_MIPS_CONSOLE_OUTPUT	8
>>> +#define KVM_HC_CLOCK_PAIRING		9
>>>  
>>>  /*
>>>   * hypercalls use architecture specific
>>>
>>>
>>>



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