linux-kernel - Re: [PATCH 01/20] x86/tdx: Introduce tdvmcall

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Message-ID: <395850c4-f8a3-46ed-9b0c-b1f47386610c@intel.com>
Date: Fri, 17 May 2024 08:21:37 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, "K. Y. Srinivasan" <kys@...rosoft.com>,
 Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>
Cc: linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-hyperv@...r.kernel.org
Subject: Re: [PATCH 01/20] x86/tdx: Introduce tdvmcall_trampoline()

On 5/17/24 07:19, Kirill A. Shutemov wrote:
> TDCALL calls are centralized into a few megawrappers that take the
> struct tdx_module_args as input. Most of the call sites only use a few
> arguments, but they have to zero out unused fields in the structure to
> avoid data leaks to the VMM. This leads to the compiler generating
> inefficient code: dozens of instructions per call site to clear unused
> fields of the structure.

I agree that this is what the silly compiler does in practice.  But my
first preference for fixing it would just be an out-of-line memset() or
a pretty bare REP;MOV.

In other words, I think this as the foundational justification for the
rest of the series leaves a little to be desired.