Re: [Xen-devel] [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

[Paolo Bonzini]

On 23/12/2014 16:14, Boris Ostrovsky wrote:
>> +    do {
>> +        version = pvti->version;
>> +
>> +        /* This is also a read barrier, so we'll read version first. */
>> +        rdtsc_barrier();
>> +        tsc = __native_read_tsc();
> 
> 
> This will cause VMEXIT on Xen with TSC_MODE_ALWAYS_EMULATE which is
> used, for example, after guest migrated (unless HW is capable of scaling
> TSC rate).

So does the __pvclock_read_cycles this is replacing (via
pvclock_get_nsec_offset).

Paolo
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