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

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

(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.
> 
> 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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