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

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Message-ID: <89a746cc-8f60-8472-0d0b-71a459e2dd64@intel.com>
Date:   Thu, 24 Nov 2022 17:18:11 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v7 16/20] x86/virt/tdx: Configure TDX module with TDMRs
 and global KeyID

On 11/24/22 16:59, Huang, Kai wrote:
> On Wed, 2022-11-23 at 15:56 -0800, Dave Hansen wrote:
>> On 11/20/22 16:26, Kai Huang wrote:
>>> +   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?
> 
> Correct.
> 
>> 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?
> 
> The alignment is assumed to be guaranteed based on the Documentation you quoted
> below.

I'm feeling kinda dense today, being Thanksgiving and all.  Could you
please walk me through, step-by-step how you get kzalloc() to give you
an allocation where the start address is 512-byte aligned?

...
> Perhaps I should just allocate one page so it must be 512-byte aligned?
> 
>         /*
>          * TDMR_INFO entries are configured to the TDX module  via an array
>          * of physical address of each TDMR_INFO.  The TDX module requires
>          * the array itself to be 512-byte aligned.  Just allocate a page
>          * to use it as the array so the alignment can be guaranteed.  The
>          * page will be freed after TDH.SYS.CONFIG anyway.
>          */

Kai, I just plain can't believe at this point that comments like this
are still being written.  I _thought_ I was very clear before that if
you have a constant, say:

#define FOO_ALIGN 512

and you want to align foo, you can just do:

	foo = ALIGN(foo, FOO_ALIGN);

You don't need to mention the 512-byte alignment again.  The #define is
good enough.