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Date:   Thu, 17 Mar 2022 21:21:41 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>, mingo@redhat.com,
        bp@alien8.de, dave.hansen@intel.com, luto@kernel.org,
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        linux-kernel@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
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Subject: Re: [PATCHv6 07/30] x86/traps: Add #VE support for TDX guest
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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.