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

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Message-ID: <53DB3517.1010408@redhat.com>
Date:	Fri, 01 Aug 2014 08:35:03 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Zhang, Yang Z" <yang.z.zhang@...el.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

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.

Paolo
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