Re: [PATCH v5 52/65] i386/tdx: Wire TDX_REPORT_FATAL_ERROR with GuestPanic facility

[Markus Armbruster]

Xiaoyao Li <xiaoyao.li@intel.com> writes:

> On 3/7/2024 9:51 PM, Markus Armbruster wrote:
>> Xiaoyao Li <xiaoyao.li@intel.com> writes:
>> 
>>> On 2/29/2024 4:51 PM, Markus Armbruster wrote:
>>>> Xiaoyao Li <xiaoyao.li@intel.com> writes:
>>>>
>>>>> Integrate TDX's TDX_REPORT_FATAL_ERROR into QEMU GuestPanic facility
>>>>>
>>>>> Originated-from: Isaku Yamahata <isaku.yamahata@intel.com>
>>>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
>>>>> ---
>>>>> Changes in v5:
>>>>> - mention additional error information in gpa when it presents;
>>>>> - refine the documentation; (Markus)
>>>>>
>>>>> Changes in v4:
>>>>> - refine the documentation; (Markus)
>>>>>
>>>>> Changes in v3:
>>>>> - Add docmentation of new type and struct; (Daniel)
>>>>> - refine the error message handling; (Daniel)
>>>>> ---
>>>>>    qapi/run-state.json   | 31 +++++++++++++++++++++--
>>>>>    system/runstate.c     | 58 +++++++++++++++++++++++++++++++++++++++++++
>>>>>    target/i386/kvm/tdx.c | 24 +++++++++++++++++-
>>>>>    3 files changed, 110 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/qapi/run-state.json b/qapi/run-state.json
>>>>> index dd0770b379e5..b71dd1884eb6 100644
>>>>> --- a/qapi/run-state.json
>>>>> +++ b/qapi/run-state.json

[...]

>>>>> @@ -564,6 +567,30 @@
>>>>>              'psw-addr': 'uint64',
>>>>>              'reason': 'S390CrashReason'}}
>>>>> +##
>>>>> +# @GuestPanicInformationTdx:
>>>>> +#
>>>>> +# TDX Guest panic information specific to TDX, as specified in the
>>>>> +# "Guest-Hypervisor Communication Interface (GHCI) Specification",
>>>>> +# section TDG.VP.VMCALL<ReportFatalError>.
>>>>> +#
>>>>> +# @error-code: TD-specific error code
>>>>> +#
>>>>> +# @message: Human-readable error message provided by the guest. Not
>>>>> +#     to be trusted.
>>>>> +#
>>>>> +# @gpa: guest-physical address of a page that contains more verbose
>>>>> +#     error information, as zero-terminated string.  Present when the
>>>>> +#     "GPA valid" bit (bit 63) is set in @error-code.
>>>>
>>>> Uh, peeking at GHCI Spec section 3.4 TDG.VP.VMCALL<ReportFatalError>, I
>>>> see operand R12 consists of
>>>>
>>>>       bits    name                        description
>>>>       31:0    TD-specific error code      TD-specific error code
>>>>                                           Panic – 0x0.
>>>>                                           Values – 0x1 to 0xFFFFFFFF
>>>>                                           reserved.
>>>>       62:32   TD-specific extended        TD-specific extended error code.
>>>>               error code                  TD software defined.
>>>>       63      GPA Valid                   Set if the TD specified 
>>>> additional
>>>>                                           information in the GPA parameter
>>>>                                           (R13).
>>>> Is @error-code all of R12, or just bits 31:0?
>>>> If it's all of R12, description of @error-code as "TD-specific error
>>>> code" is misleading.
>>>
>>> We pass all of R12 to @error_code.
>>>
>>> Here it wants to use "error_code" as generic as the whole R12. Do you have 
>>> any better description of it ?
>> 
>> Sadly, the spec is of no help: it doesn't name the entire thing, only
>> the three sub-fields TD-specific error code, TD-specific extended error
>> code, GPA valid.
>> 
>> We could take the hint, and provide the sub-fields instead:
>> 
>> * @error-code contains the TD-specific error code (bits 31:0)
>> 
>> * @extended-error-code contains the TD-specific extended error code
>>    (bits 62:32)
>> 
>> * we don't need @gpa-valid, because it's the same as "@gpa is present"
>> 
>> If we decide to keep the single member, we do need another name for it.
>> @error-codes (plural) doesn't exactly feel wonderful, but it gives at
>> least a subtle hint that it's not just *the* error code.
>
> The reason we only defined one single member, is that the 
> extended-error-code is not used now, and I believe it won't be used in 
> the near future.

Aha!  Then I recommend

* @error-code contains the TD-specific error code (bits 31:0)

* Omit bits 62:32 from the reply; if we later find an actual use for
  them, we can add a suitable member

* Omit bit 63, because it's the same as "@gpa is present"

> If no objection from others, I will use @error-codes (plural) in the 
> next version.

I recommend to keep the @error-code name, but narrow its value to the
actual error code, i.e. bits 31:0.

>>>> If it's just bits 31:0, then 'Present when the "GPA valid" bit (bit 63)
>>>> is set in @error-code' is wrong.  Could go with 'Only present when the
>>>> guest provides this information'.

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