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

From: Borislav Petkov
Date: Mon Dec 01 2014 - 17:01:08 EST

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()?

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:

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.



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