Re: [PATCH] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page

[Dave Hansen <dave.hansen@intel.com>]

On 5/19/22 11:19, Kirill A. Shutemov wrote:
>>>> The SDM has a breakdown:
>>>>
>>>> 	27.2.5 Information for VM Exits Due to Instruction Execution
>>>>
>>>> I didn't realize it came from VMREAD.  I guess I assumed it came from
>>>> some TDX module magic.  Silly me.
>>>>
>>>> The SDM makes it sound like we should be more judicious about using
>>>> 've->instr_len' though.  "All VM exits other than those listed in the
>>>> above items leave this field undefined."  Looking over
>>>> virt_exception_kernel(), we've got five cases from CPU instructions that
>>>> cause unconditional VMEXITs:
>> Ideally, what the SDM says wouldn't matter at all.  The TDX module spec really
>> should be the authorative source in this case, but it just punts to the SDM:
>>
>>   The 32-bit value that would have been saved into the VMCS as VM-exit instruction
>>   length if a legacy VM exit had occurred instead of the virtualization exception.
>>
>> Even if the TDX spec wants to punt to the SDM, it would save a lot of headache and
>> SDM reading if it also said something to the effect of:
>>
>>   The INSTRUCTION_LENGTH and INSTRUCTION_INFORMATION fields are valid for all
>>   #VEs injected by the Intel TDX Module.  The fields are undefined for #VEs
>>   injected by the CPU due to EPT Violations.
> I initiated update to the spec, but it will take time.

Understood, and thanks for doing that.

For now, let's just declare what we *expect* the spec will say and show
it to the folks doing the spec itself.  They will then have a chance to
balk at our interpretation if we got something wrong.