Re: [PATCH v11 1/3] x86/tdx: Add TDX Guest attestation interface driver

Sathyanarayanan Kuppuswamy
2022-08-29T00:19:38Z

Hi,

On 8/28/22 1:14 PM, Dave Hansen wrote:
> On 8/26/22 08:06, Kuppuswamy Sathyanarayanan wrote:
>> +struct tdx_report_req {
>> + __u8 subtype;
>> + __u64 reportdata;
>> + __u32 rpd_len;
>> + __u64 tdreport;
>> + __u32 tdr_len;
>> +};
>
> Please do take a look at how the compiler ends up building that structure.
>
> If you sized things to "save space", the way the compiler treats that
> structure may surprise you. You might also want to look at how a 32-bit
> compile deals with it versus a 64-bit one.

Since it is only used in user/kernel ABI, I did not consider the size
issue. But I understand your point. The size of this struct in a 64-bit
system is 40 bytes. So we did not gain anything with using different member
sizes. In a 32-bit system, size due to padding is less compared to 64-bit.

I will re-arrange the struct as below. With this change, the size will come
down to 32 bytes.

struct tdx_report_req {
__u64 reportdata;;
__u64 tdreport;
__u32 rpd_len
__u32 tdr_len;
__u8 subtype;
};

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer