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

[Thomas Gleixner]

On Thu, Mar 17 2022 at 21:21, Peter Zijlstra wrote:
> On Thu, Mar 17, 2022 at 08:33:54PM +0300, Kirill A. Shutemov wrote:
>>  - 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.

#VE is raised by HLT, CPUID, I/O-Port access, MSR read/write, EPT violations

So in the hairy places:

 - HLT:       No business
 - I/O Ports: That would be outright stupid to use

 - CPUID:     Should never be used - Emphasis on should :)
 - MSRs:      Same as CPUID

 - EPT:       Well....

Thanks,

        Thomas