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

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Date: Tue, 16 Apr 2024 19:45:18 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "Hansen, Dave"
	<dave.hansen@...el.com>, "Yuan, Hang" <hang.yuan@...el.com>, "x86@...nel.org"
	<x86@...nel.org>, "Huang, Kai" <kai.huang@...el.com>, "Chen, Bo2"
	<chen.bo@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "Aktas, Erdem"
	<erdemaktas@...gle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>
Subject: Re: [PATCH v19 007/130] x86/virt/tdx: Export SEAMCALL functions

On Wed, 2024-04-10 at 15:49 +0300, Kirill A. Shutemov wrote:
> On Fri, Mar 15, 2024 at 09:33:20AM -0700, Sean Christopherson wrote:
> > So my feedback is to not worry about the exports, and instead focus on
> > figuring
> > out a way to make the generated code less bloated and easier to read/debug.
> 
> I think it was mistake trying to centralize TDCALL/SEAMCALL calls into
> few megawrappers. I think we can get better results by shifting leaf
> function wrappers into assembly.
> 
> We are going to have more assembly, but it should produce better result.
> Adding macros can help to write such wrapper and minimizer boilerplate.
> 
> Below is an example of how it can look like. It's not complete. I only
> converted TDCALLs, but TDVMCALLs or SEAMCALLs. TDVMCALLs are going to be
> more complex.
> 
> Any opinions? Is it something worth investing more time?

We discussed offline how implementing these for each TDVM/SEAMCALL increases the
chances of a bug in just one TDVM/SEAMCALL. Which could making debugging
problems more challenging. Kirill raised the possibility of some code generating
solution like cpufeatures.h, that could take a spec and generate correct calls.

So far no big wins have presented themselves. Kirill, do we think the path to
move the messy part out-of-line will not work?