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

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Date:   Fri, 2 Dec 2022 11:11:40 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "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 Tue, 2022-11-22 at 15:39 -0800, Dave Hansen wrote:
> > +#define TDSYSINFO_STRUCT_SIZE		1024
> > +#define TDSYSINFO_STRUCT_ALIGNMENT	1024
> > +
> > +struct tdsysinfo_struct {
> > +	/* TDX-SEAM Module Info */
> > +	u32	attributes;
> > +	u32	vendor_id;
> > +	u32	build_date;
> > +	u16	build_num;
> > +	u16	minor_version;
> > +	u16	major_version;
> > +	u8	reserved0[14];
> > +	/* Memory Info */
> > +	u16	max_tdmrs;
> > +	u16	max_reserved_per_tdmr;
> > +	u16	pamt_entry_size;
> > +	u8	reserved1[10];
> > +	/* Control Struct Info */
> > +	u16	tdcs_base_size;
> > +	u8	reserved2[2];
> > +	u16	tdvps_base_size;
> > +	u8	tdvps_xfam_dependent_size;
> > +	u8	reserved3[9];
> > +	/* TD Capabilities */
> > +	u64	attributes_fixed0;
> > +	u64	attributes_fixed1;
> > +	u64	xfam_fixed0;
> > +	u64	xfam_fixed1;
> > +	u8	reserved4[32];
> > +	u32	num_cpuid_config;
> > +	/*
> > +	 * The actual number of CPUID_CONFIG depends on above
> > +	 * 'num_cpuid_config'.  The size of 'struct tdsysinfo_struct'
> > +	 * is 1024B defined by TDX architecture.  Use a union with
> > +	 * specific padding to make 'sizeof(struct tdsysinfo_struct)'
> > +	 * equal to 1024.
> > +	 */
> > +	union {
> > +		struct cpuid_config	cpuid_configs[0];
> > +		u8			reserved5[892];
> > +	};
> 
> Can you double check what the "right" way to do variable arrays is these
> days?  I thought the [0] method was discouraged.
> 
> Also, it isn't *really* 892 bytes of reserved space, right?  Anything
> that's not cpuid_configs[] is reserved, I presume.  Could you try to be
> more precise there?

Hi Dave,

I did some search, and I think we should use DECLARE_FLEX_ARRAY() macro?

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.