Whats the purpose of get_cycles_sync()

["Joerg Roedel" <joerg.roedel@amd.com>]

Hi,

I would like to answer what the special purpose of the get_cycles_sync()
function is in the x86 architecture. In special I ask myself why
this function has to be *sync*?

I mean, the sync should guarantee here that the CPU does not execute the
RDTSC instruction out-of-order, thats clear. But does that really
matter? If there is a cache/tlb miss before the function returns all
accuracy that should be won by the synchronous RDTSC is lost anyway.

The problem here is, that this function executes CPUID if RDTSC itself
is not a synchronizing instruction and CPUID is very often intercepted
by hypervisors (KVM intercepts it for example). This makes this function
very expensive if the kernel is executed as a guest.

But maybe I miss some important things here.

Joerg

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