[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] rdtscP and xen (and maybe the app-tsc answer I've been looking for)
On 21/09/2009 18:56, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote: > By constant_tsc I mean that CPUID:0x80000007:EDX:8 > is set. Well, if it is at least true for 99% of systems, then it might be worth enabling constant_tsc support by default, and detect TSC divergence at runtime and disbale dynamically. I think that's what Linux does (i.e., it has a fallback at runtime if its TSC assumptions turn out to be wrong). -- Keir > Upstream Linux (2.6.30) now uses the term > X86_FEATURE_TSC_RELIABLE to indicate that tsc is > consistent across cores and sockets and > X86_FEATURE_NONSTOP_TSC to indicate that it > doesn't stop in deep C-states (which Xen compensates > for) and X86_FEATURE_CONSTANT_TSC to indicate that > it stays running across P/T state transitions. > On Intel systems, CPUID:0x80000007:EDX:8 enables > all of these feature flags. (Interestingly, on > AMD systems, X86_FEATURE_TSC_RELIABLE is *not* > set by this bit... so my information from AMD is > not represented in Linux (yet)). Note also that > in linux-2.6.30/arch/x86/kernel/cpu/vmware.c, both > X86_FEATURE_CONSTANT_TSC and X86_FEATURE_TSC_RELIABLE > get set. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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