Re: [PATCH 4/5] x86/virt/tdx: Support global metadata read for all element sizes

[Isaku Yamahata]

On Sat, Mar 02, 2024 at 12:20:36AM +1300,
Kai Huang <kai.huang@xxxxxxxxx> wrote:

> For now the kernel only reads TDMR related global metadata fields for
> module initialization.  All these fields are 16-bits, and the kernel
> only supports reading 16-bits fields.
> 
> KVM will need to read a bunch of non-TDMR related metadata to create and
> run TDX guests.  It's essential to provide a generic metadata read
> infrastructure which supports reading all 8/16/32/64 bits element sizes.
> 
> Extend the metadata read to support reading all these element sizes.
> 
> Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/virt/vmx/tdx/tdx.c | 59 +++++++++++++++++++++++++------------
>  arch/x86/virt/vmx/tdx/tdx.h |  2 --
>  2 files changed, 40 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index eb208da4ff63..4ee4b8cf377c 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -271,23 +271,35 @@ static int read_sys_metadata_field(u64 field_id, u64 *data)
>  	return 0;
>  }
>  
> -static int read_sys_metadata_field16(u64 field_id,
> -				     int offset,
> -				     void *stbuf)
> +/* Return the metadata field element size in bytes */
> +static int get_metadata_field_bytes(u64 field_id)
>  {
> -	u16 *st_member = stbuf + offset;
> +	/*
> +	 * TDX supports 8/16/32/64 bits metadata field element sizes.
> +	 * TDX module determines the metadata element size based on the
> +	 * "element size code" encoded in the field ID (see the comment
> +	 * of MD_FIELD_ID_ELE_SIZE_CODE macro for specific encodings).
> +	 */
> +	return 1 << MD_FIELD_ID_ELE_SIZE_CODE(field_id);
> +}
> +
> +static int stbuf_read_sys_metadata_field(u64 field_id,
> +					 int offset,
> +					 int bytes,
> +					 void *stbuf)
> +{
> +	void *st_member = stbuf + offset;
>  	u64 tmp;
>  	int ret;
>  
> -	if (WARN_ON_ONCE(MD_FIELD_ID_ELE_SIZE_CODE(field_id) !=
> -			MD_FIELD_ID_ELE_SIZE_16BIT))
> +	if (WARN_ON_ONCE(get_metadata_field_bytes(field_id) != bytes))
>  		return -EINVAL;
>  
>  	ret = read_sys_metadata_field(field_id, &tmp);
>  	if (ret)
>  		return ret;
>  
> -	*st_member = tmp;
> +	memcpy(st_member, &tmp, bytes);
>  
>  	return 0;
>  }
> @@ -295,11 +307,30 @@ static int read_sys_metadata_field16(u64 field_id,
>  struct field_mapping {
>  	u64 field_id;
>  	int offset;
> +	int size;
>  };
>  
>  #define TD_SYSINFO_MAP(_field_id, _struct, _member)	\
>  	{ .field_id = MD_FIELD_ID_##_field_id,		\
> -	  .offset   = offsetof(_struct, _member) }
> +	  .offset   = offsetof(_struct, _member),	\
> +	  .size     = sizeof(typeof(((_struct *)0)->_member)) }

Because we use compile time constant for _field_id mostly, can we add build
time check? Something like this.

static inline metadata_size_check(u64 field_id, size_t size)
{
        BUILD_BUG_ON(get_metadata_field_bytes(field_id) != size);
}

#define TD_SYSINFO_MAP(_field_id, _struct, _member)	\
	{ .field_id = MD_FIELD_ID_##_field_id,		\
	  .offset   = offsetof(_struct, _member),	\
	  .size     = \
		({ size_t s = sizeof(typeof(((_struct *)0)->_member)); \
                metadata_size_check(MD_FIELD_ID_##_field_id, s); \
                s; }) }

-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>