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

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Date:	Mon, 1 Dec 2014 23:00:57 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, david.vrabel@...rix.com,
	konrad.wilk@...cle.com
Subject: Re: [PATCH] x86, microcode: Don't initialize microcode code on
 paravirt

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@...cle.com>
> ---
>  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()?

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?

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?

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

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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