linux-kernel - Re: [PATCH] x86/kvm/tdx: Save %rbp in TDX_MODULE

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Date: Fri, 17 May 2024 20:01:58 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Sean Christopherson <seanjc@...gle.com>, 
	Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	linux-coco@...ts.linux.dev, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/kvm/tdx: Save %rbp in TDX_MODULE_CALL

On Fri, May 17, 2024 at 09:34:56AM -0700, Dave Hansen wrote:
> On 5/17/24 09:12, Sean Christopherson wrote:
> >> There's a feature in TDX module 1.5 that prevents RBP modification across
> >> TDH.VP.ENTER SEAMCALL. See NO_RBP_MOD in TDX Module 1.5 ABI spec.
> > LOL, "feature".  How was clobbering RBP not treated as a bug?  I'm party joking,
> > but also quite serious. 
> 
> I'm on the same page.  It would have been far simpler for all involved
> to retroactively say that modifying RBP is against the rules and any
> module that does it is buggy. Get a new module if yours is buggy.
> 
> I _believe_ the intent was to support guest/host combinations that used
> RBP for whatever reason.  But I'm not sure such a combination exists or
> ever existed in practice.

There's a bug in EDK2. It specifies RBP in mask of registers to pass to
VMM. NO_RBP_MOD breaks it :/

-- 
  Kiryl Shutsemau / Kirill A. Shutemov