On Tuesday 15 January 2008 23:16:46 Avi Kivity wrote: > Alexander Graf wrote: > > If I remember correctly, there was a statement on LKML by AMD which said > > that the TSC is completely broken on AMD systems, so you should not use > > it there anyway. > > Well, that's up to the guest to decide. Looks like older Linux guests > make the wrong decision.
http://lkml.org/lkml/2005/11/4/173 That talks about AMD's TSC plans (dated 2005), but it's possible to find out whether it's a monotonically-increasing source. Basically some revisions of AMD family F onwards should support this feature: Future TSC Directions and Solutions =================================== Future AMD processors will provide a TSC that is P-state and C-State invariant and unaffected by STPCLK-throttling. This will make the TSC immune to drift. Because using the TSC for fast timer APIs is a desirable feature that helps performance, AMD has defined a CPUID feature bit that software can test to determine if the TSC is invariant. Issuing a CPUID instruction with an %eax register value of 0x8000_0007, on a processor whose base family is 0xF, returns "Advanced Power Management Information" in the %eax, %ebx, %ecx, and %edx registers. Bit 8 of the return %edx is the "TscInvariant" feature flag which is set when TSC is P-state, C-state, and STPCLK-throttling invariant; it is clear otherwise. The rate of the invariant TSC is implementation-dependent and will likely *not* be the frequency of the processor core; however, its period should be short enough such that it is not possible for two back-to-back rdtsc instructions to return the same value. Software which is trying to measure actual processor frequency or cycle-performance should use Performance Event 76h, CPU Clocks not Halted, rather than the TSC to count CPU cycles. ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
