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SubjectRe: [PATCH] KVM: x86: fix ordering of cr0 initialization code in vmx_cpu_reset
>>> On 4/28/2016 at 01:08 PM, Radim Kr*má* <> wrote: 
> 2016-04-22 12:56-0600, Bruce Rogers:
>> 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
>> initialization.
>> The combination of booting with ovmf firmware when guest vcpus > 1 and kvm's
>> ept=N option being set results in a VM-entry failure. This patch fixes that.
> Greg pointed out missing,
> Cc:
> when was Cc'd. Adding
> Fixes: d28bc9dd25ce ("KVM: x86: INIT and reset sequences are different")
> would be nice too (even when it is redundant).
>> Signed-off-by: Bruce Rogers <>
>> ---
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> @@ -5046,8 +5046,8 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool
> init_event)
>> 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 */
> So vmx_set_cr0() has a code that depends on vmx->vcpu.arch.cr0 being
> already set the to new value. Do you know what function is it?

The one which I partly referred to above is the following:
vmx_set_cr0() ->
enter_rmode() ->
kvm_mmu_reset_context() ->
init_kvm_softmmu() ->
kvm_init_shadow_mmu() ->
which uses the CR0 WP bit.
There may be other problematic references. I haven't tried to do an
exhaustive search.

> I think we better set vmx->vcpu.arch.cr0 early in vmx_set_cr0().
> Or do other callsites somehow depend on the old cr0 value?

You may be right that that is a better overall fix. I was simply trying
to undo the erroneous lines in the commit which broke things, partly
to have a patch better suited for applying to stable releases.

I'll send a v2 shortly with your suggested additions to the patch header.



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