linux-kernel - Re: [PATCH v8 08/16] x86/virt/tdx: Add placeholder to construct TDMRs to cover all TDX memory regions

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Message-ID: <56cdb6e6-e25c-aba9-7bb3-323281e65249@intel.com>
Date:   Tue, 10 Jan 2023 11:12:13 -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>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "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 v8 08/16] x86/virt/tdx: Add placeholder to construct TDMRs
 to cover all TDX memory regions

On 1/9/23 18:23, Huang, Kai wrote:
> On Mon, 2023-01-09 at 16:47 -0800, Dave Hansen wrote:
>> On 1/9/23 16:40, Huang, Kai wrote:
>>> On Fri, 2023-01-06 at 11:24 -0800, Dave Hansen wrote:
>> ...
>>>> Also, tdmr_sz and max_tdmrs can both be derived from 'sysinfo'.  Do they
>>>> really need to be stored here?
>>>
>>> It's not mandatory to keep them here.  I did it mainly because I want to avoid
>>> passing 'sysinfo' as argument for almost all functions related to constructing
>>> TDMRs.
>>
>> I don't think it hurts readability that much.  On the contrary, it makes
>> it more clear what data is needed for initialization.
> 
> Sorry one thing I forgot to mention is if we keep 'tdmr_sz' in 'struct
> tdmr_info_list', it only needs to be calculated at once when allocating the
> buffer.  Otherwise, we need to calculate it based on sysinfo-
> max_reserved_per_tdmr each time we want to get a TDMR at a given index.

What's the problem with recalculating it?  It is calculated like this:

	tdmr_sz = ALIGN(constant1 + constant2 * variable);

So, what's the problem?  You're concerned about too many multiplications?

> To me putting relevant fields (tdmrs, tdmr_sz, max_tdmrs, nr_consumed_tdmrs)
> together makes how the TDMR list is organized more clear.  But please let me
> know if you prefer removing 'tdmr_sz' and 'max_tdmrs'.
> 
> Btw, if we remove 'tdmr_sz' and 'max_tdmrs', even nr_consumed_tdmrs is not
> absolutely necessary here.  It can be a local variable of init_tdx_module() (as
> shown in v7), and the 'struct tdmr_info_list' will only have the 'tdmrs' member
> (as you commented in v7):
> 
> https://lore.kernel.org/linux-mm/cc195eb6499cf021b4ce2e937200571915bfe66f.camel@intel.com/T/#mb9826e2bcf8bf6399c13cc5f95a948fe4b3a46d9
> 
> Please let me know what's your preference?

I dunno.  My gut says that passing sysinfo around and just deriving the
sizes values from that with helpers is the best way.  'struct
tdmr_info_list' isn't a horrible idea in and of itself, but I think it's
a confusing structure because it's not clear how the pieces fit together
when half of it is *required* and the other half is just for some kind
of perceived convenience.