linux-kernel - Re: [PATCH v8 0/9] TDX host: metadata reading tweaks and bug fixes

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Message-ID: <45a8ac0d-9f79-405a-80a4-40f5886c3bde@intel.com>
Date: Thu, 14 Nov 2024 11:40:55 +1300
From: "Huang, Kai" <kai.huang@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "Hansen, Dave"
	<dave.hansen@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"bp@...en8.de" <bp@...en8.de>, "peterz@...radead.org" <peterz@...radead.org>,
	"hpa@...or.com" <hpa@...or.com>, "mingo@...hat.com" <mingo@...hat.com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "Williams, Dan J" <dan.j.williams@...el.com>
CC: "nik.borisov@...e.com" <nik.borisov@...e.com>, "Hunter, Adrian"
	<adrian.hunter@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, "Yamahata,
 Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH v8 0/9] TDX host: metadata reading tweaks and bug fixes


> So I think it is not part of the "bare minimum". I don't have any objection with
> it going upstream with rest of this series if it doesn't delay it. But I want to
> make sure we don't have any more confusion that will cause further delays.

We have two issues that need to be addressed.  Addressing them could 
bring the infrastructure that is needed for KVM TDX as well, so this is 
the "minimal code" given the goal I want to achieve here.

> 
>> 2) Some old modules can clobber host's RBP when existing from the TDX
>>     guest, and currently they can be initialized successfully.  We don't
>>     want to use such modules thus we should just fail to initialize them
>>     to avoid memory/cpu cycle cost of initializing TDX module [2].
> 
> I think we need RBP MOD for basic support, or it may cause crashes when we start
> booting TDs.
> 
> Does all that seem correct?

We will need additional patch to save/restore RBP.  The more important 
thing is it's naturally bad, due to the thing that I mentioned in that 
patch:

"...clobbering RBP could result in bad things like being unable to 
unwind the stack if any non-maskable exceptions (NMI, #MC etc) happens 
in that gap."