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

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Message-ID: <551d2539-91b4-41e1-97e6-701921e83db7@intel.com>
Date: Fri, 17 May 2024 09:34:56 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: 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 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.