Re: [edk2-devel] [PATCH 12/17] OvmfPkg/PvScsiDxe: Reset adapter on init

["Liran Alon" <liran.alon@oracle.com>]

On 25/03/2020 18:40, Liran Alon wrote:
>
> On 25/03/2020 18:31, Laszlo Ersek wrote:
>> On 03/25/20 02:11, Liran Alon wrote:
>>> To avoid further style comments, what is the coding convention in EDK2
>>> to align the "PVSCSI_CMD_DESC_SETUP_RINGS Cmd;" var properly?
>> The best I can recommend off-hand is:
>>
>> union {
>>    PVSCSI_CMD_DESC_SETUP_RINGS Cmd;
>>    UINT32                      Uint32;
>> } AlignAtUint32;
>>
>> Perhaps someone else can recommend something less awkward.
>>
>> Note: PVSCSI_CMD_DESC_SETUP_RINGS is a packed structure (and I do agree
>> that's good, if at least for documentation purposes). If it weren't
>> packed, then the following passage from the UEFI spec would apply:
>>
>>      2.3.1 Data Types
>>
>>      Table 5 lists the common data types that are used in the interface
>>      definitions, and Table 6 lists their modifiers. Unless otherwise
>>      specified all data types are naturally aligned. Structures are
>>      aligned on boundaries equal to the largest internal datum of the
>>      structure and internal data are implicitly padded to achieve 
>> natural
>>      alignment.
>>
>> Because PVSCSI_CMD_DESC_SETUP_RINGS only contains members with types
>> listed in Table 5 (namely, UINT32 and UINT64), the above language would
>> normally guarantee the proper alignment. *But*, because the structure is
>> packed, I don't think we can rely on the spec's description (cf. "unless
>> otherwise specified").
>>
>> So, in theory, there are two options:
>>
>> - drop the packing (and rely on the natural alignment providing what you
>>    need anyway),
>>
>> - keep the packing, and use other methods to guarantee struct-level
>>    alignment (such as the above union).
>>
>> I prefer keeping the packing, if for nothing else then for documentation
>> purposes (it says "wire format" loud and clear). If you use the union
>> above, I'll be OK with it.
> Ok. I will use this union approach.
> Unfortunately, I have seen this comment only after submitting v2.
> So I will wait for the rest of your v2 review comments and make sure 
> to do this change for v3 as-well.
>
> Thanks.
>
Actually, I'm not sure I understand how this union approach helps with 
anything.
Isn't the PVSCSI_CMD_DESC_SETUP_RINGS structure already aligned because 
it have only UINT32 and UINT64 fields?
And if alignment is not guaranteed, how does putting it in a union 
together with another UINT32 provides the required alignment it didn't 
had before?
Because the union itself is not marked with packed(), in contrast to 
PVSCSI_CMD_DESC_SETUP_RINGS?

-Liran