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RE: [Xen-devel] Question about time measure in Guest Windows

Thanks for your suggest!
But I have a further question about this issue: In my tests, I need to use some other instructions like RDTSC that may cause VMExit conditionally, one example is WBINVD which is another bit in VM-execution control field. From your advice, can I say that I can ignore all the bits of VM-execution control field in Xen? Otherwise, How to query VM-Execution control field in a programmable way?


> Date: Thu, 4 Dec 2008 07:10:36 -0800
> From: dan.magenheimer@xxxxxxxxxx
> To: nanxuan_xu@xxxxxxx
> Subject: RE: [Xen-devel] Question about time measure in Guest Windows
> "RDTSC exiting VM execution control field" is available so that
> a hypervisor can trap ALL guest uses of the RDTSC instruction.
> Xen does not currently use this, so you can use RDTSC in a Xen
> Windows guest without worrying about it.
> HOWEVER, be aware that on many SMP machines, the TSC is not
> synchronized between processors and your guest may move between
> processors without its knowledge. So two consecutive reads
> of TSC by a guest may appear as if TSC is going backwards or
> jumps forward by many many ticks.
> -----Original Message-----
> From: XuNanxuan [mailto:nanxuan_xu@xxxxxxx]
> Sent: Wednesday, December 03, 2008 11:45 PM
> To: xen-devel@xxxxxxxxxxxxxx e.com
> Subject: [Xen-devel] Question about time measure in Guest Windows
> Hi:
> I intend to use RDTSC to make some time tests in Xen guest WinXP. However, from Intel Developer's Manual I noticed that RDTSC will cause VMExit if "RETSC exiting" VM-Execution control field is 1. So I wonder: How to query VM-Execution control field in a programmable way?
> BTW: On Intel Developer's Manual, it mentioned a method using IA32_VMX_PROCBASED_CTLS MSR, but this is just to report which bits can be set on processor-based VM-EXECUTION controls, rather than telling you the content of this structure.
> Thanks!

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