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

Radim Krcmar
2017-01-13T15:31:58Z

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
>
>