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

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Message-ID: <20141201215725.GG24289@laptop.dumpdata.com>
Date:	Mon, 1 Dec 2014 16:57:25 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>, gleb@...nel.org,
	pbonzini@...hat.com, kvm@...r.kernel.org
Cc:	bp@...e.de, x86@...nel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, david.vrabel@...rix.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.

CC-ing the KVM folks since they use the paravirt interface too.
> 
> 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)
>  		return 0;
>  
>  	if (c->x86_vendor == X86_VENDOR_INTEL)
> -- 
> 1.7.1
> 
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