linux-kernel - Re: [PATCH v19 086/130] KVM: TDX: restore debug store when TD exit

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Date: Wed, 17 Apr 2024 12:03:15 +0800
From: Chao Gao <chao.gao@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
CC: <isaku.yamahata@...el.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <isaku.yamahata@...il.com>, Paolo Bonzini
	<pbonzini@...hat.com>, <erdemaktas@...gle.com>, Sean Christopherson
	<seanjc@...gle.com>, Sagi Shahar <sagis@...gle.com>, Kai Huang
	<kai.huang@...el.com>, <chen.bo@...el.com>, <hang.yuan@...el.com>,
	<tina.zhang@...el.com>
Subject: Re: [PATCH v19 086/130] KVM: TDX: restore debug store when TD exit

On Tue, Apr 16, 2024 at 11:24:16AM -0700, Reinette Chatre wrote:
>Hi Isaku,
>
>On 2/26/2024 12:26 AM, isaku.yamahata@...el.com wrote:
>> From: Isaku Yamahata <isaku.yamahata@...el.com>
>> 
>> Because debug store is clobbered, restore it on TD exit.
>> 
>
>I am trying to understand why this is needed and finding a few
>places that seem to indicate that this is not needed.
>
>For example, in the TX base spec, section 15.2 "On-TD Performance
>Monitoring", subsection "15.2.1 Overview":
> * IA32_DS_AREA MSR is context-switched across TD entry and exit transitions.
>
>To confirm I peeked at the TDX module code and found (if I understand this
>correctly) that IA32_DS_AREA is saved on TD entry (see [1]) and restored on 
>TD exit (see [2]).

Hi Reinette,

You are right. I asked TDX module to preserve IA32_DS_AREA across TD transitions
and they made this change. So, this patch can be dropped now.