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

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Date:	Wed, 13 Jul 2016 17:54:24 +0800
From:	Yang Zhang <yang.zhang.wz@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP

On 2016/7/13 17:35, Paolo Bonzini wrote:
>
>
> 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.

You are right. And SGDT, SIDT, SLDT, SMSW, STR also will be intercepted 
even in CPL 0 if we don't disable it.

-- 
Yang
Alibaba Cloud Computing