Re: [edk2-devel] [PATCH v2 5/5] UefiCpuPkg/MpInitLib: Add support for multiple HOBs to SaveCpuMpData()

["Laszlo Ersek" <lersek@redhat.com>]

On 2/22/24 02:49, Laszlo Ersek wrote:
> (commenting once more, to explain the actual reason:)
> 
> On 2/21/24 11:24, Gerd Hoffmann wrote:
>> On Wed, Feb 21, 2024 at 03:48:25AM +0000, Ni, Ray wrote:
>>>>
>>>> +  MaxCpusPerHob = (MAX_UINT16 - sizeof (EFI_HOB_GUID_TYPE) - sizeof
>>>> (MP_HAND_OFF)) / sizeof (PROCESSOR_HAND_OFF);
>>>
>>> Above formula assumes the maximum HOB length could be 0xFFFF.
>>
>> Which is IMHO correct.
> 
> It is not correct:
> 
>>
>>> But actually the maximum HOB length could be only 0xFFF8 because
>>> PeiCore::PeiCreateHob() contains following logic:
>>>
>>>   if (0x10000 - Length <= 0x7) {
>>>     return EFI_INVALID_PARAMETER;
>>>   }
>>
>> That Length is the *data* size, the HOB header is not included.

sorry, I meant to insert here: no; "Length" at this location does
include everything, except the padding.

The BuildGuidHob() library function takes a data length, and calculates
the total HOB length, by adding 24 bytes, for the EFI_HOB_GUID_TYPE header.

InternalPeiCreateHob() (also in "MdePkg/Library/PeiHobLib/HobLib.c")
already takes the total HOB length, and passes it on to the CreateHob()
PEI service -- which Ray quotes above.

The total HOB length cannot exceed 0xFFF8 (due to the rounding up, for
Itanium's sake), so the GUID HOB data length cannot exceed
(0xFFF8-sizeof(EFI_HOB_GUID_TYPE)).

Laszlo

>>
>> The "- sizeof (EFI_HOB_GUID_TYPE)" in the formula above accounts the
>> space needed for HOB header and GUID.
> 
> Yes, but the problem is not that we need the extra space for
> EFI_HOB_GUID_TYPE (24 bytes) -- we need the extra space for padding /
> alignment.
> 
> This is what the PI spec says (v1.8, section "III-4.5.2 HOB Construction
> Rules"):
> 
>   HOB construction must obey the following rules:
> 
>   [...]
> 
>   4. All HOBs must be multiples of 8 bytes in length. This requirement
>      meets the alignment restrictions of the Itanium® processor family.
> 
>   [...]
> 
> And if your total *desired* HOB length exceeds 0xFFF8 (i.e., it falls in
> [0xFFF9..0xFFFF]), then the required upwards rounding produces 0x1_0000.
> But that rounded-up value cannot be expressed in
> "EFI_HOB_GENERIC_HEADER.HobLength" -- a UINT16 field.
> 
> In short, you've got Itanium to thank for this. :)
> 
> Laszlo



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