RE: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Jeremy Fitzhardinge
>Sent: 2009年8月5日 8:06
>
>On 07/24/09 01:04, Keir Fraser wrote:
>> Okay, so the issue you are worried about is not specific to 
>Xen. So how is
>> native Linux tackling this, for example?
>>   
>
>Linux will use the tsc where possible, but regularly assesses its
>perceived accuracy and will move to a different clocksource if the tsc
>appears to the playing up.  I don't think it ever assumes the tsc is
>synced between CPU/cores.

It cares. See tsc_sync.c under x86 arch, where unsynced warps
mark tsc as unstable. 

Thanks,
Kevin

>
>It allows rdtsc from usermode, but it is generally considered 
>to be very
>buggy and ill-defined behaviour.  It makes no attempt to make usermode
>rdtsc in any way meaningful.  The exception is the vgettimeofday
>vsyscall which does Xen-like timekeeping, in which it gets the tsc,cpu
>tuple atomically, then scales it with timing parameters from 
>the kernel.
>
>    J
>
>_______________________________________________
>Xen-devel mailing list
>Xen-devel@xxxxxxxxxxxxxxxxxxx
>http://lists.xensource.com/xen-devel
>

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel