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

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Message-ID: <e5ebcab0d1104765ced1fafebf737b7c311593a3.camel@intel.com>
Date: Wed, 13 Nov 2024 22:53:00 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Hansen, Dave" <dave.hansen@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "bp@...en8.de" <bp@...en8.de>, "Huang, Kai"
	<kai.huang@...el.com>, "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

On Thu, 2024-11-14 at 11:40 +1300, Huang, Kai wrote:
> > 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.

I'm confused by this. "could bring the infrastructure"? What is the "goal I want
to achieve"?

Let me ask it another way, if we drop patches 7 and 8 and pushed them in a later
series. (say after TDX gets upstream for the sake of this question, but I'm not
suggesting a schedule). Then what is the consequence to the goal of booting a TD
on some HW?

I'm not questioning that the patches are in order, or that they should be fixed
urgently. I'm just trying to make sure we are clear to Dave on why this is all
needed.

> 
> > 
> > > 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.  

I think you are talking about the workaround patch that was NAKed and we have
now dropped from the dev branch since we can now rely on NO_RBP_MOD? So we
*don't* need that patch...?

> 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."
> 
> 

Yea I think NO_RBP_MOD is required.