Re: [PATCH v7 16/20] x86/virt/tdx: Configure TDX module with TDMRs and global KeyID

From: Dave Hansen <dave.hansen@intel.com>
To: Kai Huang <kai.huang@intel.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc: linux-mm@kvack.org, seanjc@google.com, pbonzini@redhat.com,
	dan.j.williams@intel.com, rafael.j.wysocki@intel.com,
	kirill.shutemov@linux.intel.com, ying.huang@intel.com,
	reinette.chatre@intel.com, len.brown@intel.com,
	tony.luck@intel.com, peterz@infradead.org, ak@linux.intel.com,
	isaku.yamahata@intel.com, chao.gao@intel.com,
	sathyanarayanan.kuppuswamy@linux.intel.com, bagasdotme@gmail.com,
	sagis@google.com, imammedo@redhat.com
Subject: Re: [PATCH v7 16/20] x86/virt/tdx: Configure TDX module with TDMRs and global KeyID
Date: Wed, 23 Nov 2022 15:56:54 -0800	[thread overview]
Message-ID: <301184ce-05e5-871c-7a6c-4298a0cbd1ae@intel.com> (raw)
In-Reply-To: <344234642a5eb9dc1aa34410f641f596ec428ea5.1668988357.git.kai.huang@intel.com>

On 11/20/22 16:26, Kai Huang wrote:
> After the TDX-usable memory regions are constructed in an array of TDMRs
> and the global KeyID is reserved, configure them to the TDX module using
> TDH.SYS.CONFIG SEAMCALL.  TDH.SYS.CONFIG can only be called once and can
> be done on any logical cpu.
> 
> Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> ---
>  arch/x86/virt/vmx/tdx/tdx.c | 37 +++++++++++++++++++++++++++++++++++++
>  arch/x86/virt/vmx/tdx/tdx.h |  2 ++
>  2 files changed, 39 insertions(+)
> 
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index e2cbeeb7f0dc..3a032930e58a 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -979,6 +979,37 @@ static int construct_tdmrs(struct tdmr_info *tdmr_array, int *tdmr_num)
>  	return ret;
>  }
>  
> +static int config_tdx_module(struct tdmr_info *tdmr_array, int tdmr_num,
> +			     u64 global_keyid)
> +{
> +	u64 *tdmr_pa_array;
> +	int i, array_sz;
> +	u64 ret;
> +
> +	/*
> +	 * TDMR_INFO entries are configured to the TDX module via an
> +	 * array of the physical address of each TDMR_INFO.  TDX module
> +	 * requires the array itself to be 512-byte aligned.  Round up
> +	 * the array size to 512-byte aligned so the buffer allocated
> +	 * by kzalloc() will meet the alignment requirement.
> +	 */

Aagh.  Return of (a different) 512-byte aligned structure.

> +	array_sz = ALIGN(tdmr_num * sizeof(u64), TDMR_INFO_PA_ARRAY_ALIGNMENT);
> +	tdmr_pa_array = kzalloc(array_sz, GFP_KERNEL);

Just to be clear, all that chatter about alignment is because the
*START* of the array has to be aligned.  Right?  I see alignment for
'array_sz', but that's not the start of the array.

tdmr_pa_array is the start of the array.  Where is *THAT* aligned?

How does rounding up the size make kzalloc() magically know how to align
the *START* of the allocation?

Because I'm actually questioning my own sanity at this point, I went and
double-checked the docs (Documentation/core-api/memory-allocation.rst):

> The address of a chunk allocated with `kmalloc` is aligned to at least
> ARCH_KMALLOC_MINALIGN bytes.  For sizes which are a power of two, the
> alignment is also guaranteed to be at least the respective size.

Hint #1: ARCH_KMALLOC_MINALIGN is way less than 512.
Hint #2: tdmr_num is not guaranteed to be a power of two.
Hint #3: Comments don't actually affect the allocation

<snip>