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: <cba4c7295b157dc5d0d88b73f335899f0aa5a21c.camel@intel.com>
Date:   Tue, 10 Jan 2023 02:23:34 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "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 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.

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?

> 
> > > If so, I think I'd probably do something
> > > like this with the structure:
> > > 
> > > struct tdmr_info_list {
> > >       struct tdmr_info *tdmrs;
> > >       int nr_consumed_tdmrs; // How many @tdmrs are in use
> > > 
> > >       /* Metadata for freeing this structure: */
> > >       int tdmr_sz;   // Size of one 'tdmr_info' (has a flex array)
> > >       int max_tdmrs; // How many @tdmrs are allocated
> > > };
> > > 
> > > Modulo whataver folks are doing for comments these days.
> > 
> > Looks nice to me.  Will use.  A slight thing is 'tdmr_sz' is also used to get
> > the TDMR at a given index, but not just freeing the structure.
> > 
> > Btw, is C++ style comment "//" OK in kernel code?
> 
> It's OK with me, but I don't think there's much consensus on it.
> Probably best to stick with normal arch/x86 style for now.
> 
> 

Will use normal arch/x86 style for now.  Thanks for the info.