Re: [PATCH] x86, microcode: Don't initialize microcode code on paravirt

[Boris Ostrovsky]

On 12/01/2014 05:00 PM, Borislav Petkov wrote:
> On Mon, Dec 01, 2014 at 04:27:44PM -0500, Boris Ostrovsky wrote:
> > Paravirtual guests are not expected to load microcode into processors
> > and therefore it is not necessary to initialize microcode loading
> > logic.
> >
> > In fact, under certain circumstances initializing this logic may cause
> > the guest to crash. Specifically, 32-bit kernels use __pa_nodebug()
> > macro which does not work in Xen (the code path that leads to this macro
> > happens during resume when we call mc_bp_resume()->load_ucode_ap()
> > ->check_loader_disabled_ap())
> >
> > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > ---
> >   arch/x86/kernel/cpu/microcode/core.c |    2 +-
> >   1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
> > index 2ce9051..ebd232d 100644
> > --- a/arch/x86/kernel/cpu/microcode/core.c
> > +++ b/arch/x86/kernel/cpu/microcode/core.c
> > @@ -557,7 +557,7 @@ static int __init microcode_init(void)
> >   	struct cpuinfo_x86 *c = &cpu_data(0);
> >   	int error;
> >   
> > -	if (dis_ucode_ldr)
> > +	if (paravirt_enabled() || dis_ucode_ldr)
> Ok, let me make sure I understand this correctly: The early path doesn't
> get executed on paravirt, i.e. the path along load_ucode_intel_ap()?

(+KVM folks here as well)

Xen PV doesn't start with startup_32()/startup_32_smp() so for Xen this 
is true. Don't know about KVM (or lguest).

> And you want to avoid loading of the microcode driver because the only
> path we come to load_ucode_ap() on paravirt is the hotplug notifier?

That's my understanding, yes.

> Btw, we've applied another fix today for 3.18 final which limits the
> microcode reloading to 64-bit only:
>
> http://git.kernel.org/tip/02ecc41abcea4ff9291d548f6f846b29b354ddd2
>
> which should accidentally fix the paravirt issue too, no?

I think so. The problem we have now is __pa() macro that we only use on 
32-bit. I'll queue this for overnight tests to make sure and if it 
indeed works then 3.19 should be fine.

Thanks.
-boris

> Because if so, I'd like to delay your patch for the 3.19 merge window
> unless absolutely necessary.
>
> Thanks.

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