Hi Andi,
On Tue, Oct 30, 2007 at 09:21:02PM +0100, Andi Kleen wrote:
> "Joerg Roedel" <[email protected]> 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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
