linux-kernel - Re: [PATCH] KVM: nVMX: nested TPR shadow/threshold emulation

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Date:	Thu, 31 Jul 2014 11:25:37 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Jan Kiszka <jan.kiszka@...mens.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...nel.org>, Bandan Das <bsd@...hat.com>,
	Zhang Yang <yang.z.zhang@...el.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: nested TPR shadow/threshold emulation

Il 31/07/2014 10:03, Wanpeng Li ha scritto:
>> One thing:
>>
>>> +	if (nested_cpu_has(vmcs12, CPU_BASED_TPR_SHADOW))
>>> +		vmcs_write32(TPR_THRESHOLD, vmcs12->tpr_threshold);
>>
>> I think you can just do this write unconditionally, since most
>> hypervisors will enable this.  Also, you probably can add the tpr
> 
> What will happen if a hypervisor doesn't enable it? I make it more 
> cleaner in version two.

TPR_THRESHOLD will be likely written as zero, but the processor will
never use it anyway.  It's just a small optimization because
nested_cpu_has(vmcs12, CPU_BASED_TPR_SHADOW) will almost always be true.

Paolo

>> threshold field to the read-write fields for shadow VMCS.
> 
> Agreed.
> 
> Regards,
> Wanpeng Li 

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