linux-kernel - Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP

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Message-ID: <b28bc6d9-ce39-e024-c6f1-51b756c72e45@redhat.com>
Date:	Wed, 13 Jul 2016 11:35:10 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Yang Zhang <yang.zhang.wz@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP



On 13/07/2016 11:21, Yang Zhang wrote:
>>
>> +    if ((cr4 & X86_CR4_UMIP) && !boot_cpu_has(X86_FEATURE_UMIP)) {
>> +        vmcs_set_bits(SECONDARY_VM_EXEC_CONTROL,
>> +                  SECONDARY_EXEC_DESC);
>> +        hw_cr4 &= ~X86_CR4_UMIP;
>> +    } else
>> +        vmcs_clear_bits(SECONDARY_VM_EXEC_CONTROL,
>> +                SECONDARY_EXEC_DESC);
>> +
> 
> Since the faults based on privilege level have priority over VM exits.
> So we don't need to enable/disable SECONDARY_EXEC_DESC dynamically.
> Instead, we can set it unconditionally.

I'm setting it dynamically because it slows down LGDT, LLDT, LIDT and LTR.

Paolo