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

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Date:   Mon, 29 Aug 2022 07:30:00 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Shuah Khan <shuah@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kai Huang <kai.huang@...el.com>,
        Wander Lairson Costa <wander@...hat.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v11 1/3] x86/tdx: Add TDX Guest attestation interface
 driver

On Sun, Aug 28, 2022 at 05:19:33PM -0700, Sathyanarayanan Kuppuswamy wrote:
> 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.

That is _EXACTLY_ why you need to consider the size issues here.

> 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.

That's not the issue here, please use a tool like pahole to see the
problems with this definition.

> 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;
> };

That's better, but again, please use pahole.

thanks,

greg k-h