linux-kernel - Re: [PATCHv6 07/30] x86/traps: Add #VE support for TDX guest

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Date:   Thu, 17 Mar 2022 21:21:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...el.com, luto@...nel.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCHv6 07/30] x86/traps: Add #VE support for TDX guest

On Thu, Mar 17, 2022 at 08:33:54PM +0300, Kirill A. Shutemov wrote:

> [ Disclaimer: I have limited understanding of the entry code complexity
>   and may miss some crucial details. But I try my best. ]
> 
> Yes, it is the same comment, but it is based on code audit, not only on
> testing.
> 
> I claim that kernel does not do anything that can possibly trigger #VE
> where kernel cannot deal with it:
> 
>  - on syscall entry code before kernel stack is set up (few instructions
>    in the beginning of entry_SYSCALL_64())
> 
>  - in NMI entry code (asm_exc_nmi()) before NMI nesting is safe:
>    + for NMI from user mode, before switched to thread stack
>    + for NMI from kernel, up to end_repead_nmi
> 
> After that points #VE is safe.

In what way is it guaranteed that #VE isn't raised in those places? What
does an auditor / future coder looking to changes things, need to
consider to keep this so.

>From vague memories #VE can be raised on any memop, loading the stack
address in the syscall-gap is a memop. What makes that special? Can we
get a comment _there_ to explain how this is safe such that we can keep
it so?

Same for the NMI path I suppose.