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

[Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>]

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