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Subject: Re: [RFC v2-fix 1/1] x86/tdx: Handle in-kernel MMIO
From:   Andi Kleen <ak@linux.intel.com>
To:     Dave Hansen <dave.hansen@intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Tony Luck <tony.luck@intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        linux-kernel@vger.kernel.org
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Message-ID: <b5089180-1b73-cd1b-98a2-c3cc624d2e5c@linux.intel.com>
Date:   Tue, 18 May 2021 10:28:00 -0700
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On 5/18/2021 9:10 AM, Andi Kleen wrote:
>
>>>>> For now we only handle a subset of instructions that the kernel
>>>>> uses for MMIO operations. User-space access triggers SIGBUS.
>>>> How do you know which instructions the kernel uses?
>>> They're all in MMIO macros.
>> I've heard exactly the opposite from the TDX team in the past. What I
>> remember was a claim that one can not just leverage the MMIO macros as a
>> single point to avoid MMIO.  I remember being told that not all code in
>> the kernel that does MMIO uses these macros.  APIC MMIO's were called
>> out as a place that does not use the MMIO macros.
>
> Yes x86 APIC has its own macros, but we don't use the MMIO based APIC, 
> only X2APIC in TDX.

I must correct myself here. We actually use #VE to handle MSRs, or at 
least those that are not context switched by the TDX module. So there 
can be #VE nested in NMI in normal operation, since MSR accesses in NMI 
can happen.

I don't think it needs any changes to the code -- this should all work 
-- but we need to update the commit log to document this case.


-Andi