linux-kernel - Re: [PATCH v4 3/8] x86/virt/tdx: Prepare to support reading other global metadata fields

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Message-ID: <408dee3f-a466-4746-92d3-adf54d35ec7c@intel.com>
Date: Thu, 26 Sep 2024 08:47:54 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Kai Huang <kai.huang@...el.com>, kirill.shutemov@...ux.intel.com,
 tglx@...utronix.de, bp@...en8.de, peterz@...radead.org, mingo@...hat.com,
 hpa@...or.com, dan.j.williams@...el.com, seanjc@...gle.com,
 pbonzini@...hat.com
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 rick.p.edgecombe@...el.com, isaku.yamahata@...el.com,
 adrian.hunter@...el.com, nik.borisov@...e.com
Subject: Re: [PATCH v4 3/8] x86/virt/tdx: Prepare to support reading other
 global metadata fields

On 9/24/24 04:28, Kai Huang wrote:
> +#define build_sysmd_read(_size)							\
> +static int __read_sys_metadata_field##_size(u64 field_id, u##_size *val)	\
> +{										\
> +	u64 tmp;								\
> +	int ret;								\
> +										\
> +	ret = tdh_sys_rd(field_id, &tmp);					\
> +	if (ret)								\
> +		return ret;							\
> +										\
> +	*val = tmp;								\
> +										\
> +	return 0;								\
>  }

Why?  What's so important about having the compiler do the copy?


#define read_sys_metadata_field(id, val) 	\
	__read_sys_metadata_field(id, val, sizeof (*(val)))

static int __read_sys_metadata_field(u64 field_id, void *ptr, int size)
{
	...
	memcpy(ptr, &tmp, size);

	return 0;
}

There's one simple #define there so that users don't have to do the
sizeof and can't screw it up.