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

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Date:	Fri, 1 Aug 2014 06:44:10 +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 2014-08-01:
> Il 01/08/2014 02:57, Zhang, Yang Z ha scritto:
>>> 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.
> 
> Yes, you do need an "if (cpu_has_vmx_tpr_shadow())" around the
> vmcs_write32.  But still, checking nested_cpu_has is not strictly necessary.
> Right now they both are a single AND, but I have plans to change all
> of the
> cpu_has_*() checks to static keys.

See v2 patch. It isn't a problem anymore.

> 
> Paolo


Best regards,
Yang


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