linux-kernel - Re: [PATCH 1/2] KVM: x86: fix ordering of cr0 initialization code in vmx_cpu

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Date:	Mon, 08 Feb 2016 09:29:30 -0700
From:	"Bruce Rogers" <brogers@...e.com>
To:	"Paolo Bonzini" <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Cc:	<namit@...technion.ac.il>, <jan.kiszka@...mens.com>
Subject: Re: [PATCH 1/2] KVM: x86: fix ordering of cr0 initialization
 code in vmx_cpu_reset

>>> On 2/8/2016 at 08:09 AM, Paolo Bonzini <pbonzini@...hat.com> wrote: 

> 
> On 03/02/2016 23:51, Bruce Rogers wrote:
>> 
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index e2951b6..21507b4 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -4993,8 +4993,8 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool 
> init_event)
>>  		vmcs_write16(VIRTUAL_PROCESSOR_ID, vmx->vpid);
>>  
>>  	cr0 = X86_CR0_NW | X86_CR0_CD | X86_CR0_ET;
>> -	vmx_set_cr0(vcpu, cr0); /* enter rmode */
>>  	vmx->vcpu.arch.cr0 = cr0;
>> +	vmx_set_cr0(vcpu, cr0); /* enter rmode */
> 
> Your comment that the assignment is redundant is correct, but I am
> afraid that this fix is also wrong.  In particular, it would not cause
> exit_lmode and enter_rmode to be called.
> 
> You are not describing which call to kvm_mmu_reset_context was messed
> up, so I'm not sure how your patch is fixing things.

This is in the context of AP sending INIT to BSP with unrestricted_guest=N.

So the call sequence where I see the issue is: kvm_apic_accept_events() ->
kvm_vcpu_reset() -> vmx_vcpu_reset() -> vmx_set_cr0() -> enter_rmode() ->
kvm_mmu_reset_context().

enter_rmode is called in the case I am testing.

Bruce