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

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Date:	Mon, 8 Feb 2016 16:05:47 +0000
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Borislav Petkov <bp@...en8.de>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
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 15:55, Borislav Petkov wrote:
> On Mon, Feb 08, 2016 at 10:39:43AM -0500, Boris Ostrovsky wrote:
>> It does. Very much IIRC, the problem was not caused by an access to MSR but
>> rather some sort of address not being available somewhere.
> See below.
>
>>> - microcode application on Xen: we've had this before. The hypervisor
>>> should do that (if it doesn't do so already).
>> it does.
> Good.
>
>>> So yes, that paravirt_enabled() thing should go away. Even more so if we
>>> have CPUID leaf 0x4... reserved for hypervisors.
>> I actually think this was the original proposal until we realized we had
>> paravirt_enabled(). So we can go back to checking CPUID 0x40000000.
>>
>> We might also be able to test for (x86_hyper!=NULL) and have guests that do
>> microcode management prior to init_hypervisor() rely on hypervisors ignoring
>> MSR accesses (as they do today).
> Right, so the early loader can't do that as on 32-bit it runs even
> before paging has been enabled. So I *think* the thing with CPUID would
> be best. What does the xen hypervisor return in regs when I do CPUID(4)?
> I.e., how do I reliably detect it in the guest?
>
> I can whip up a quick patch and get rid of paravirt_enabled() while at
> it...
>

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.

~Andrew