Re: Whats the purpose of get_cycles_sync()

[Joerg Roedel <joro@8bytes.org>]

Hi Andi,

On Tue, Oct 30, 2007 at 09:21:02PM +0100, Andi Kleen wrote:
> "Joerg Roedel" <joerg.roedel@amd.com> writes:
> 
> > 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*?
> 
> Vojtech had one test that tested time monotonicity over CPUs 
> and it constantly failed until we added the CPUID on K8 C stepping. 
> He can give details on the test.

Interesting, I wasn't aware of that.

> I suspect the reason was because the CPU reordered the RDTSCs so that
> a later RDTSC could return a value before an earlier one. This can
> happen because gettimeofday() is so fast that a tight loop calling it can
> fit more than one iteration into the CPU's reordering window.

Ok, that is the reason why the get_cycles_sync() function only exists on
x86_64 and not on i386, because on i386 gettimeofday() is a real
syscall?

> That is why newer kernels use RDTSCP if available which doesn't need
> to be intercepted and is synchronous.  And since all AMD SVM systems
> have RDTSCP they are fine.

The problem with KVM here is that they wan't to migrate guests between
Intel and AMD boxes. So they don't export RDTSCP or FEATURE_SYNC_TSC to
the guests in the CPUID calls. A 64bit Linux guest will execute the
CPUID in that function.

Joerg