linux-kernel - Re: [PATCH v19 007/130] x86/virt/tdx: Export SEAMCALL functions

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Date: Fri, 15 Mar 2024 01:17:07 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Hansen, Dave"
	<dave.hansen@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Huang, Kai" <kai.huang@...el.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "x86@...nel.org"
	<x86@...nel.org>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "Yuan, Hang" <hang.yuan@...el.com>, "Chen, Bo2"
	<chen.bo@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "Aktas, Erdem"
	<erdemaktas@...gle.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v19 007/130] x86/virt/tdx: Export SEAMCALL functions

Dave, care to weigh in here?

Context (whether to export the generic __seamcall for KVM's use):
https://lore.kernel.org/lkml/e6e8f585-b718-4f53-88f6-89832a1e4b9f@intel.com/

On Fri, 2024-03-15 at 13:02 +1300, Huang, Kai wrote:
> KVM roughly will need to use dozens of SEAMCALLs, and all these are 
> logically related to creating and running TDX guests.  It makes more 
> sense to just export __seamcall() and let KVM maintain these VM-
> related 
> wrappers rather than having the TDX host code to provide wrappers for
> each SEAMCALL or higher-level abstraction.

The helpers being discussed are these:
https://lore.kernel.org/lkml/7cfd33d896fce7b49bcf4b7179d0ded22c06b8c2.1708933498.git.isaku.yamahata@intel.com/

I guess there are three options:
1. Export the low level seamcall function
2. Export a bunch of higher level helper functions
3. Duplicate __seamcall asm in KVM

Letting modules make unrestricted seamcalls is not ideal. Preventing
the compiler from inlining the small logic in the static inline helpers
is not ideal. Duplicating code is not ideal. Hmm.

I want to say 2 sounds the least worst of the three. But I'm not sure.
I'm not sure if x86 folks would like to police new seamcalls, or be
bothered by it, either.