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

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Date: Fri, 17 May 2024 17:48:11 +0200
From: Juergen Gross <jgross@...e.com>
To: Dave Hansen <dave.hansen@...el.com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: 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 17.05.24 17:43, Dave Hansen wrote:
> On 5/17/24 08:27, Jürgen Groß wrote:
>> On 17.05.24 17:16, Dave Hansen wrote:
>>> On 5/17/24 07:44, Juergen Gross wrote:
>>>> Just another data point: Before using this machine I was testing on
>>>> another one with older firmware. That one really didn't support
>>>> NOM_RBP_MOD
>>>> and I needed to build the kernel with CONFIG_FRAME_POINTER enabled to
>>>> get
>>>> past the check you are mentioning above.
>>>
>>> For all intents and purposes, the modules that intentionally clobber RBP
>>> don't support Linux. If buggy modules are accidentally clobbering RBP,
>>> we can debate how much the kernel should bend over to accommodate them,
>>> but my preference would be to ignore them.
>>>
>>> I'd much rather put a deny list in the kernel than try to tolerate RBP
>>> clobbering universally.
>>
>> Would you be fine with adding a new X86_FEATURE (or BUG?) allowing
>> to switch RBP save/restore via ALTERNATIVE, controlled by a command
>> line option?
>>
>> Or maybe by adding a new CONFIG_TDX_MODULE_CAN_CLOBBER_RBP (probably
>> using a shorter name) option?
> 
> As a last resort maybe.
> 
>> TBH I'm slightly puzzled that the firmware I'm using could make it
>> outside Intel. I'm fearing this might happen again.
> 
> You're puzzled that the firmware is either old buggy or both? Huh.
> 
> Intel ships all kinds of crazy pre-production stuff as development
> platforms. Let's make sure we know what you've got before we go tearing
> up mainline for it.
> 
> Because if the options are:
> 
>   1. Maintain code in mainline until the day I die^Wretire
> 
> or
> 
>   2. Get Jürgen a BIOS update so he stops sending patches
> 
> .. it's kinda an easy choice. ;)
> 

:-)

Is the BIOS version printed at boot enough to see what I have?

[    0.000000] DMI: Intel Corporation D50DNP/D50DNP, BIOS 
SE5C7411.86B.9535.D04.2312270518 12/27/2023


Juergen

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