Re: [patch 2/3] KVM: x86: add KVM_HC_CLOCK_OFFSET hypercall

[Radim Krcmar <rkrcmar@redhat.com>]

2017-01-13 10:01-0200, Marcelo Tosatti:
> 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 |   30 ++++++++++++++++++++++
>  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, 83 insertions(+)
> 
> 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 09:03:33.823305270 -0200
> +++ kvm-ptpdriver/arch/x86/include/uapi/asm/kvm_para.h	2017-01-13 09:07:54.633699270 -0200
> @@ -50,6 +50,15 @@
>  	__u32 pad[11];
>  };
>  
> +#define KVM_CLOCK_OFFSET_WALLCLOCK 0
> +struct kvm_clock_offset {
> +	__s64 sec;
> +	__s64 nsec;
> +	__u64 tsc;
> +	__u32 flags;
> +	__u32 pad[1];
> +};
> +
>  #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 09:03:33.823305270 -0200
> +++ kvm-ptpdriver/Documentation/virtual/kvm/hypercalls.txt	2017-01-13 09:15:32.971389949 -0200
> @@ -81,3 +81,33 @@
>  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_OFFSET
> +------------------------
> +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_OFFSET_WALLCLOCK (0)
> +is supported (hosts CLOCK_REALTIME clock).
> +
> +		struct kvm_clock_offset {
> +			__s64 sec;
> +			__s64 nsec;

Why is nsec:
 1) signed -- it is a remainder after division by NSEC_PER_SEC
 2) bigger than 32 bit -- NSEC_PER_SEC < 2^32
?

> +			__u64 tsc;
> +			__u32 flags;
> +			__u32 pad;
> +		};
> +
> +       Where:
> +               * sec: seconds from clock_type clock.
> +               * nsec: nanoseconds from clock_type clock.

The important part of an offset is the starting point -- I assume it is
the the usual one, but documentation better be explicit.

> +               * tsc: TSC value used to calculate sec/nsec pair

This hypercall could in theory work even when computing the offset with
a different clocksource on the host and the estimating TSC at that time.

> +                 (this hypercall only works when host uses TSC clocksource).

Describe that error as a return value.

> +               * flags: flags, unused at the moment.

unused = 0.

> +
> Index: kvm-ptpdriver/arch/x86/kvm/x86.c
> ===================================================================
> --- kvm-ptpdriver.orig/arch/x86/kvm/x86.c	2017-01-13 09:04:46.581415259 -0200
> +++ kvm-ptpdriver/arch/x86/kvm/x86.c	2017-01-13 09:08:51.557785166 -0200
> @@ -6121,6 +6121,44 @@
>  }
>  EXPORT_SYMBOL_GPL(kvm_emulate_halt);
>  
> +static int kvm_pv_clock_offset(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_OFFSET_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);

Can't clock_offset be on the stack?

> +	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.
>   *
> @@ -6185,6 +6223,9 @@
>  		kvm_pv_kick_cpu_op(vcpu->kvm, a0, a1);
>  		ret = 0;
>  		break;
> +	case KVM_HC_CLOCK_OFFSET:
> +		ret = kvm_pv_clock_offset(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 09:03:33.823305270 -0200
> +++ kvm-ptpdriver/include/uapi/linux/kvm_para.h	2017-01-13 09:07:54.636699274 -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
>  
>  #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_OFFSET		9
>  
>  /*
>   * hypercalls use architecture specific
> 
>