linux-kernel - Re: [PATCH 3/9] x86/virt/tdx: Support global metadata read for all element sizes

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Date: Wed, 19 Jun 2024 11:05:19 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org
Cc: x86@...nel.org, dave.hansen@...el.com, dan.j.williams@...el.com,
 kirill.shutemov@...ux.intel.com, rick.p.edgecombe@...el.com,
 peterz@...radead.org, tglx@...utronix.de, bp@...en8.de, mingo@...hat.com,
 hpa@...or.com, seanjc@...gle.com, pbonzini@...hat.com, kvm@...r.kernel.org,
 isaku.yamahata@...el.com, binbin.wu@...ux.intel.com
Subject: Re: [PATCH 3/9] x86/virt/tdx: Support global metadata read for all
 element sizes



On 16.06.24 г. 15:01 ч., Kai Huang wrote:
> The TDX module provides "global metadata fields" for software to query.
> Each metadata field is accessible by a unique "metadata field ID".  TDX
> supports 8/16/32/64 bits metadata element sizes.  The size of each
> metadata field is encoded in its associated metadata field ID.
> 
> For now the kernel only reads "TD Memory Region" (TDMR) related global
> metadata fields for module initialization.  All these metadata fields
> are 16-bit, and the kernel only supports reading 16-bit fields.
> 
> Future changes will need to read more metadata fields with other element
> sizes.  To resolve this once for all, extend the existing metadata
> reading code to support reading all element sizes.
> 
> Signed-off-by: Kai Huang <kai.huang@...el.com>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>   arch/x86/virt/vmx/tdx/tdx.c | 29 ++++++++++++++++-------------
>   arch/x86/virt/vmx/tdx/tdx.h |  3 ++-
>   2 files changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index 854312e97eff..4392e82a9bcb 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -270,23 +270,25 @@ 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)
> +/*
> + * Read one global metadata field and store the data to a location of a
> + * given buffer specified by the offset and size (in bytes).
> + */
> +static int stbuf_read_sysmd_field(u64 field_id, void *stbuf, int offset,
> +				  int bytes)

Actually I think opencoding read_sys_metadata_field in 
stbuf_read_sysmd_field and leaving the function named as
read_sys_metadata_field would be best. The new function is still very 
short and linear.

Another point - why do proliferate the offset calculations in the lower 
layers of TDX. Simply pass in a buffer and a size, calculate the exact 
location in the callers of the read functions.

<snip>