linux-kernel - Re: [Xen-devel] [PATCH v2 3/3] paravirt: rename paravirt_enabled to paravirt

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Date:	Mon, 8 Feb 2016 16:26:36 +0000
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Borislav Petkov <bp@...en8.de>
CC:	X86 ML <x86@...nel.org>, <david.e.box@...el.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Lv Zheng <lv.zheng@...el.com>, Jan Beulich <JBeulich@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>, <qiuxishi@...wei.com>,
	<cocci@...teme.lip6.fr>, Xen Devel <xen-devel@...ts.xensource.com>,
	"Joerg Roedel" <joro@...tes.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Robert Moore <robert.moore@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	"Andrey Ryabinin" <aryabinin@...tuozzo.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>,
	Thomas Gleixner <tglx@...utronix.de>, <mcb30@...e.org>,
	Juergen Gross <jgross@...e.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Andy Lutomirski" <luto@...capital.net>,
	"Luis R. Rodriguez" <mcgrof@...nel.org>,
	<long.wanglong@...wei.com>, Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [Xen-devel] [PATCH v2 3/3] paravirt: rename paravirt_enabled to
 paravirt_legacy

On 08/02/16 16:12, Boris Ostrovsky wrote:
>
>
> On 02/08/2016 11:05 AM, Andrew Cooper wrote:
>>
>> For compatibility with other virtualisation specs, Xen's cpuid leaves
>> shift depending on configuration.
>>
>> Spec at
>> http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/arch-x86/cpuid.h;h=d709340f18d089560b959835eabb7b6609542c7e;hb=HEAD#l33
>>
>>
>> Basically, they are either at 0x40000000, or 0x40000100 if viridian or
>> vmware compatibility has been enabled.
>
> (Also in arch/x86/include/asm/xen/cpuid.h in Linux tree).

The other complication is that, for a PV guest, you must use the forced
emulation prefix for the cpuid instruction to trap into the hypervisor.

This either means you must be sure you are under Xen to start with, or
use an extable entry to catch the #UD fault if you are not under Xen.

~Andrew