linux-kernel - Re: [PATCH v7 09/20] x86/virt/tdx: Get information about TDX module and TDX-capable memory

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Message-ID: <21b43adc-37aa-bac3-0615-4703438ea4a1@intel.com>
Date:   Fri, 2 Dec 2022 09:06:04 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v7 09/20] x86/virt/tdx: Get information about TDX module
 and TDX-capable memory

On 12/2/22 03:11, Huang, Kai wrote:
> And also to address you concern that not all 892 bytes are reserved, how about
> below:
> 
>         union {
> -               struct cpuid_config     cpuid_configs[0];
> -               u8                      reserved5[892];
> +               DECLARE_FLEX_ARRAY(struct cpuid_config, cpuid_configs);
> +               u8 padding[892];
>         };
>  } __packed __aligned(TDSYSINFO_STRUCT_ALIGNMENT);
> 
> The goal is to make the size of 'struct tdsysinfo_struct' to be 1024B so we can
> use a static variable for it, and at the meantime, it can still have 1024B
> (enough space) for the TDH.SYS.INFO to write to.

I just don't like the open-coded sizes.

For instance, wouldn't it be great if you didn't have to know the size
of *ANYTHING* else to properly size the '892'?

Maybe we just need some helpers to hide the gunk:

#define DECLARE_PADDED_STRUCT(type, name, alignment) 	\
struct type##_padded {					\
	union {						\
		struct type name;			\
		u8 padding[alignment];			\
	}						\
} name##_padded;

#define PADDED_STRUCT(name)	(name##_padded.name)

That can get used like this:

DECLARE_PADDED_STRUCT(struct tdsysinfo_struct, tdsysinfo,
		      TDSYSINFO_STRUCT_ALIGNMENT);


	struct tdsysinfo_struct sysinfo = PADDED_STRUCT(tdsysinfo)