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

[Marcelo Tosatti]

On Fri, Jan 13, 2017 at 04:31:58PM +0100, Radim Krcmar wrote:
> 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@xxxxxxxxxx>
> > 
> > ---
> >  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

Because "struct timespec" is signed because it can be used for
time deltas (you won't actually get signed values for
kvm_get_walltime_and_clockread).

Just wanted to match "struct timespec".

>  2) bigger than 32 bit -- NSEC_PER_SEC < 2^32
> ?

Again matching struct timespec.

> > +			__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.

Don't get what you mean? (the values have same meaning as hosts
clock_gettime(CLOCK_REALTIME), supposedly that is clear).

> > +               * 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.

Yes it could.

> > +                 (this hypercall only works when host uses TSC clocksource).
> 
> Describe that error as a return value.

Fixed.

+
+       if (kvm_get_walltime_and_clockread(&ts, &cycle) == false) {
+               kfree(clock_offset);
+               return -KVM_EOPNOTSUPP;
+       }

> 
> > +               * flags: flags, unused at the moment.
> 
> unused = 0.

Fixed.

> > +
> > 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?

Well since the kernel stack is limited, thought it would be safer to 
allocate it.

> > +	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
> > 
> >