pv clock: kvm is incompatible with xen :-(

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

Tried to use kvmclock with xenner and noticed that the kvmclock
(MSR_KVM_SYSTEM_TIME msr) is incompatible with xen.

kvm guests do this to translate the tsc delta into nsecs:

   #define get_clock(cpu, field) per_cpu(hv_clock, cpu).field

   static inline u64 kvm_get_delta(u64 last_tsc)
   {
           int cpu = smp_processor_id();
           u64 delta = native_read_tsc() - last_tsc;
           return (delta * get_clock(cpu, tsc_to_system_mul)) >> 22;
   }

whereas xen guests do this (64bit version):

   static inline u64 scale_delta(u64 delta, u32 mul_frac, int shift)
   {
           u64 product;

           if (shift < 0)
                   delta >>= -shift;
           else
                   delta <<= shift;

           __asm__ (
                   "mul %%rdx ; shrd $32,%%rdx,%%rax"
                   : "=a" (product) : "0" (delta), "d" ((u64)mul_frac));
           return product;
   }

Note that xen does a 64bit multiply (of the 64bit delta and the 32bit
factor) yielding a 128bit result, then picking bits 32-95 for the 64bit
return value.  In contrast kvm does a simple 64bit multiply, which is
equivalent to using the lowest 64 bits.  Thus kvm is off by factor 2^32,
and that without even considering the ordering of two (shift + multiply)
operations and any rounding errors ...

cheers,
  Gerd

-- 
http://kraxel.fedorapeople.org/xenner/

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