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RE: [Xen-users] Dom0 mask CPU flags?

To: "bupt.cc" <bupt.cc@xxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] Dom0 mask CPU flags?
From: "Jonathan Tripathy" <jonnyt@xxxxxxxxxxx>
Date: Thu, 12 Aug 2010 09:12:53 +0100
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Thread-topic: [Xen-users] Dom0 mask CPU flags?
Yup, it sure does.
I got bitten by this a while ago. I was starting to get worried that my CPU didn't support some features, but in fact Xen was just masking the flags. This is a security feature.
I can't find it on the wiki, but it does mention it somewhere there.
Remember that the Xen Hypervisor itself it sitting on top of the CPU, so it doesn't really matter too much what flags are shown in a guest (Dom0 is a guest btw), as long as the kernel can understand specific requests and "proxy" them correctly

From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx on behalf of bupt.cc
Sent: Thu 12/08/2010 08:00
To: xen-users
Subject: [Xen-users] Dom0 mask CPU flags?

Hi all,
Does Xen dom0 mask CPU flags or what?
I execute "cat /proc/cpuinfo" on bare-metal linux ,and get:
| flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36
|          clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt rdtscp lm
|          3dnowext 3dnow rep_good extd_apicid pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy
But when I boot into Xen dom0 linux, I get:
|flags : fpu de tsc msr pae mce cx8 apic mtrr mca cmov pat clflush mmx fxsr sse sse2 
|            ht syscall nx mmxext fxsr_opt lm 3dnowext 3dnow rep_good pni cx16 
|            hypervisor lahf_lm cmp_legacy extapic cr8_legacy
Couple of flags are missing. 
How does that happen?
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