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

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Date:	Fri, 1 Aug 2014 00:57:11 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	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>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] KVM: nVMX: nested TPR shadow/threshold emulation

Paolo Bonzini wrote on 2014-07-31:
> 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.

Theoretically, you are right. But we should not expect all VMMs follow it. It is not worth to violate the SDM just for saving two or three instructions' cost.

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


Best regards,
Yang


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