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

[Huang, Kai]

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.