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

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Message-ID: <20160206220437.GA4435@pd.tnic>
Date:	Sat, 6 Feb 2016 23:04:37 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Luis R. Rodriguez" <mcgrof@...e.com>,
	"Luis R. Rodriguez" <mcgrof@...nel.org>, cocci@...teme.lip6.fr,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Juergen Gross <jgross@...e.com>, mcb30@...e.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Joerg Roedel <joro@...tes.org>,
	Robert Moore <robert.moore@...el.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jan Beulich <JBeulich@...e.com>, Lv Zheng <lv.zheng@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	long.wanglong@...wei.com, Fengguang Wu <fengguang.wu@...el.com>,
	qiuxishi@...wei.com, Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	david.e.box@...el.com, X86 ML <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 3/3] paravirt: rename paravirt_enabled to
 paravirt_legacy

On Sat, Feb 06, 2016 at 12:05:32PM -0800, Andy Lutomirski wrote:
> int __init microcode_init(void)
> {
>         [...]
>         if (paravirt_enabled() || dis_ucode_ldr)
>                 return -EINVAL;
> 
> This is also asking "are we the natively booted kernel?"  This is
> plausibly useful for real.  (Borislav, is this actually necessary?)

There was some breakage on 32-bit pvops with that.

> Seems to me there should be a function is_native_root_kernel() or
> similar.  Obviously it could have false positives and code will have
> to deal with that.  (This also could be entirely wrong.  What code is
> responsible for CPU microcode updates on Xen?  For all I know, dom0 is
> *supposed* to apply microcode updates, in which case that check really
> should be deleted.

So there are two aspects:

- the guest loading the microcode driver. Xen should behave like
qemu+kvm does: emulate the MSR accesses the microcode loader does.

- microcode application on Xen: we've had this before. The hypervisor
should do that (if it doesn't do so already).

So yes, that paravirt_enabled() thing should go away. Even more so if we
have CPUID leaf 0x4... reserved for hypervisors.

-- 
Regards/Gruss,
    Boris.

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