linux-kernel - Re: Can't boot as Xen dom0 due to commit fe055896

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Date:   Fri, 16 Dec 2016 08:28:46 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...e.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>
Subject: Re: Can't boot as Xen dom0 due to commit fe055896

On 15/12/16 18:36, Borislav Petkov wrote:
> On Thu, Dec 15, 2016 at 12:27:49PM -0500, Boris Ostrovsky wrote:
>> It will probably fix it but I don't think we want this: it's a
>> build-time solution. Most kernels have XEN on even though they are
>> booted bare-metal.
> 
> Lemme tell you want I want: a way to detect I'm running on xen. Does
> CPUID(4) work really early, at load_ucode_bsp() time?
> 
> IOW, can I use some of the functionality hypervisor_cpuid_base() uses to
> detect xen and stop loading any further?

What you really need is to avoid being called on a Xen pv guest. And
this is easy by using xen_domain().

Not trying to load ucode in _any_ guest is an optimization only.

The attached patch works for me in dom0, bare metal and Xen HVM guest.


Juergen

View attachment "0001-x86-microcode-don-t-try-to-load-microcode-when-runni.patch" of type "text/x-patch" (1404 bytes)