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

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Date:	Fri, 22 Apr 2016 12:55:31 -0600
From:	"Bruce Rogers" <brogers@...e.com>
To:	"Nadav Amit" <nadav.amit@...il.com>
Cc:	"Nadav Amit" <namit@...technion.ac.il>, <jan.kiszka@...mens.com>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: x86: fix ordering of cr0 initialization
 code in vmx_cpu_reset

>>> On 2/3/2016 at 04:18 PM, Nadav Amit <nadav.amit@...il.com> wrote: 
> Oops.
> 
> Anyhow, I see my patch has done a similar change in init_vmcb() , so you may
> want to revert it as well.
> 
> Nadav
> 
> Bruce Rogers <brogers@...e.com> wrote:
> 
>> Commit d28bc9dd25ce reversed the order of two lines which initialize cr0,
>> allowing the current (old) cr0 value to mess up vcpu initialization.
>> This was observed in the checks for cr0 X86_CR0_WP bit in the context of
>> kvm_mmu_reset_context().  Besides, setting vcpu->arch.cr0 after vmx_set_cr0()
>> is completely redundant. Change the order back to ensure proper vcpu
>> intiialization.
>> 
>> Signed-off-by: Bruce Rogers <brogers@...e.com>
>> ---
>> arch/x86/kvm/vmx.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> 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 */
>> 	vmx_set_cr4(vcpu, 0);
>> 	vmx_set_efer(vcpu, 0);
>> 	vmx_fpu_activate(vcpu);
>> -- 
>> 1.9.0
> 

I had not pursued this as the initial problem I was chasing ended up including some
undefined behavior.

Since, I've run into another failure which this patch addresses (ovmf based booting with
vcpu count >1 on older hardware), so I'll resend this one patch with updated info.

Also, it seems to me  that the init_vmcb() svm issue Nadav mentioned is no longer an
issue in the current master branch so I won't be addressing that.

Bruce