Re: [PATCH v2] kvmclock: Ensure time in migration never goes backward

[Paolo Bonzini <pbonzini@redhat.com>]

Il 16/05/2014 17:15, Alexander Graf ha scritto:
> When we migrate we ask the kernel about its current belief on what the guest
> time would be. However, I've seen cases where the kvmclock guest structure
> indicates a time more recent than the kvm returned time.
>
> To make sure we never go backwards, calculate what the guest would have seen
> as time at the point of migration and use that value instead of the kernel
> returned one when it's more recent.
>
> While the underlying bug is supposedly fixed on newer KVM versions, it doesn't
> hurt to base the view of the kvmclock after migration on the same foundation
> in host as well as guest.
>
> Signed-off-by: Alexander Graf <agraf@suse.de>
>
> ---
>
> v1 -> v2:
>
>   - always use guest structure when available
> ---
>  hw/i386/kvm/clock.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 48 insertions(+)
>
> diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> index 892aa02..6f4ed28a 100644
> --- a/hw/i386/kvm/clock.c
> +++ b/hw/i386/kvm/clock.c
> @@ -14,6 +14,7 @@
>   */
>
>  #include "qemu-common.h"
> +#include "qemu/host-utils.h"
>  #include "sysemu/sysemu.h"
>  #include "sysemu/kvm.h"
>  #include "hw/sysbus.h"
> @@ -34,6 +35,47 @@ typedef struct KVMClockState {
>      bool clock_valid;
>  } KVMClockState;
>
> +struct pvclock_vcpu_time_info {
> +    uint32_t   version;
> +    uint32_t   pad0;
> +    uint64_t   tsc_timestamp;
> +    uint64_t   system_time;
> +    uint32_t   tsc_to_system_mul;
> +    int8_t     tsc_shift;
> +    uint8_t    flags;
> +    uint8_t    pad[2];
> +} __attribute__((__packed__)); /* 32 bytes */
> +
> +static uint64_t kvmclock_current_nsec(KVMClockState *s)
> +{
> +    CPUState *cpu = first_cpu;
> +    CPUX86State *env = cpu->env_ptr;
> +    hwaddr kvmclock_struct_pa = env->system_time_msr & ~1ULL;
> +    uint64_t migration_tsc = env->tsc;
> +    struct pvclock_vcpu_time_info time;
> +    uint64_t delta;
> +    uint64_t nsec_lo;
> +    uint64_t nsec_hi;
> +    uint64_t nsec;
> +
> +    if (!(env->system_time_msr & 1ULL)) {
> +        /* KVM clock not active */
> +        return 0;
> +    }
> +
> +    cpu_physical_memory_read(kvmclock_struct_pa, &time, sizeof(time));
> +
> +    delta = migration_tsc - time.tsc_timestamp;
> +    if (time.tsc_shift < 0) {
> +        delta >>= -time.tsc_shift;
> +    } else {
> +        delta <<= time.tsc_shift;
> +    }
> +
> +    mulu64(&nsec_lo, &nsec_hi, delta, time.tsc_to_system_mul);
> +    nsec = (nsec_lo >> 32) | (nsec_hi << 32);
> +    return nsec + time.system_time;
> +}
>
>  static void kvmclock_vm_state_change(void *opaque, int running,
>                                       RunState state)
> @@ -45,9 +87,15 @@ static void kvmclock_vm_state_change(void *opaque, int running,
>
>      if (running) {
>          struct kvm_clock_data data;
> +        uint64_t time_at_migration = kvmclock_current_nsec(s);
>
>          s->clock_valid = false;
>
> +	/* We can't rely on the migrated clock value, just discard it */
> +	if (time_at_migration) {
> +	        s->clock = time_at_migration;
> +	}
> +
>          data.clock = s->clock;
>          data.flags = 0;
>          ret = kvm_vm_ioctl(kvm_state, KVM_SET_CLOCK, &data);
>

I'm going to revert this patch for 2.1-rc3, since the dependent patch 
"kvmclock: Ensure proper env->tsc value for kvmclock_current_nsec 
calculation" causes a hang during migration.

Paolo