From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [Xen-devel] [PATCH v2 3/3] paravirt: rename paravirt_enabled to
 paravirt_legacy
Date: Mon, 8 Feb 2016 16:05:47 +0000
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To: Borislav Petkov <bp@alien8.de>, Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: X86 ML <x86@kernel.org>, david.e.box@intel.com, Andrey Ryabinin <ryabinin.a.a@gmail.com>, Lv Zheng <lv.zheng@intel.com>, Jan Beulich <JBeulich@suse.com>, "H. Peter Anvin" <hpa@zytor.com>, qiuxishi@huawei.com, cocci@systeme.lip6.fr, Xen Devel <xen-devel@lists.xensource.com>, Joerg Roedel <joro@8bytes.org>, "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>, Robert Moore <robert.moore@intel.com>, Ingo Molnar <mingo@redhat.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Mauro Carvalho Chehab <mchehab@osg.samsung.com>, "Luis R. Rodriguez" <mcgrof@suse.com>, Rusty Russell <rusty@rustcorp.com.au>, Thomas Gleixner <tglx@linutronix.de>, mcb30@ipxe.org, Juergen Gross <jgross@suse.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@amacapital.net>, "Luis R. Rodriguez" <mcgrof@kernel.org>, long.wanglong@huawei.com, Fenggu
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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