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Date:   Wed, 25 Jan 2017 10:55:05 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Radim Krcmar <rkrcmar@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 2/4] KVM: x86: add KVM_HC_CLOCK_OFFSET hypercall

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@...hat.com>
> > 
> > ---
> >  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).

> >  #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.

> > +		};
> > +
> > +       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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