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

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Date:	Mon, 8 Feb 2016 15:45:21 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Borislav Petkov <bp@...en8.de>,
	Andrew Cooper <andrew.cooper3@...rix.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 02/08/2016 11:52 AM, Boris Ostrovsky wrote:
>
>
> On 02/08/2016 11:45 AM, Borislav Petkov wrote:
>> On Mon, Feb 08, 2016 at 04:38:40PM +0000, Andrew Cooper wrote:
>>> Does the early loader have extable support?  If so, this is fairly easy
>>> to fix.  If not, we have a problem.
>> It doesn't and regardless, you want to have this CPUID querying as
>> simple as possible. No special handling, no special prefixes as it
>> should be able to run on other hypervisors too.
>>
>> If one can't execute a simple CPUID(0x4...) on a xen guest and get the
>> results back, then for early, we will have to do what we've done until
>> now and simply emulate the MSR accesses.
>
> I think xen_hypervisor check can be done in microcode_init() as this 
> is first time PV kernel deals with microcode.
>
> Let me try it --- I want to see what happens on hotplug and resume but 
> I am reasonably certain this should work during boot.

So it looks like we can just simply revert a18a0f6850 because the very 
next patch to microcode code (fbae4ba8c4a) makes the original problem 
(of using __pa_nodebug, which we shouldn't use on PV) go away: we don't 
call load_ucode_ap from resume path anymore.

-boris