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

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Message-ID: <547CE7D6.4030601@redhat.com>
Date:	Mon, 01 Dec 2014 23:12:38 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>, gleb@...nel.org,
	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 01/12/2014 22:57, Konrad Rzeszutek Wilk 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.
> 
> CC-ing the KVM folks since they use the paravirt interface too.

We also do not want to load microcode. :)  Thanks for the heads-up.

Acked-by: Paolo Bonzini <pbonzini@...hat.com>

Paolo

>> 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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