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

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Message-ID: <56B8C7EC.3090403@citrix.com>
Date:	Mon, 8 Feb 2016 16:53:00 +0000
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	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 16:45, 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.
>
> Later code can use then xen_cpuid() and all is fine. We should still get
> rid of paravirt_enabled() though.
>

The force emulation prefix starts with a ud2a instruction, so extable is
to prevent it breaking on non-Xen systems.  However, if extable isn't
available, this point is moot.

As an alternative check which should be doable this early on, peeking in
the head of hypercall_page should work.  If Linux was booted as a PV
guest, the hypercall_page will have been constructed by the domain
builder, and won't have 0x90's in it.

~Andrew