Re: [PATCH v3 08/26] x86/virt/seamldr: Retrieve P-SEAMLDR information

[Dave Hansen]

On 1/29/26 20:01, Xu Yilun wrote:
>> I'd also prefer a
>>
>> 	BUILD_BUG_ON(sizeof(struct seamldr_info) != 2048);
>                                                     ^
> BUILD_BUG_ON(sizeof(struct seamldr_info) != 256);   is it?
Whatever the documentation says. I might have been looking at the
seamldr_seaminfo.

>> just as a sanity check. It doesn't cost anything and it makes sure that
>> as you muck around with reserved fields and padding that there's at
>> least one check making sure it's OK.
> 
> And I recently received a comments that "never __packed for naturally
> aligned structures cause it leads to bad generated code and hurts
> performance", but I really want to highlight nearby it is for a
> formatted binary blob, so:
> 
>   struct seamldr_info {
> 	u32     version;
> 	u32     attributes;
> 	u32     vendor_id;
> 	u32     build_date;
> 	u16     build_num;
> 	u16     minor_version;
> 	u16     major_version;
> 	u16     update_version;
> 	u8      reserved0[4];
> 	u32     num_remaining_updates;
> 	u8      reserved1[224];
>   };   //delete __packed here
> 
>  static_assert(sizeof(struct seamldr_info) == 256);
> 
> Is it better?

I'm pretty sure __packed is used all over the place.

I'd be shocked if access to a __packed structure generated different
code than a non-packed one for the same layout. But it wouldn't be the
first time I was shocked by a compiler.

I think you might be confusing the fact that access to unaligned data
can really stink on some architectures. The code generation for *that*
can be garbage. But not on x86 really and not for data that's already
naturally aligned.

Plus, *this* data structure is far, far from being performance sensitive
anyway. So it doubly or triply doesn't matter here.

If nothing else, __packed is a good indicator that WYSIWYG for structure
layout because it's an ABI. I honestly don't see a lot of downsides.