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

[Paolo Bonzini <pbonzini@redhat.com>]

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@redhat.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];
> +};
> +
>  #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.

> +		};
> +
> +       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.

> +               * 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.

> +	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.

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?

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
> 
> 
>