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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     mingo@redhat.com, bp@alien8.de, dave.hansen@intel.com,
        luto@kernel.org, sathyanarayanan.kuppuswamy@linux.intel.com,
        aarcange@redhat.com, ak@linux.intel.com, dan.j.williams@intel.com,
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        wanpengli@tencent.com, thomas.lendacky@amd.com,
        brijesh.singh@amd.com, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [PATCHv6 07/30] x86/traps: Add #VE support for TDX guest
In-Reply-To: <20220317202141.GO8939@worktop.programming.kicks-ass.net>
References: <20220316020856.24435-1-kirill.shutemov@linux.intel.com>
 <20220316020856.24435-8-kirill.shutemov@linux.intel.com>
 <877d8t2ykp.ffs@tglx> <20220317173354.rqymufl37lcrtmjh@black.fi.intel.com>
 <20220317202141.GO8939@worktop.programming.kicks-ass.net>
Date:   Fri, 18 Mar 2022 15:19:34 +0100
Message-ID: <87a6dncphl.ffs@tglx>
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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