linux-kernel - Re: [PATCH v11 10/20] x86/virt/tdx: Add placeholder to construct TDMRs to cover all TDX memory regions

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Message-ID: <127f95b84355e8c08f314a0db56e958ed8ef0636.camel@intel.com>
Date:   Thu, 8 Jun 2023 10:18:31 +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>,
        "david@...hat.com" <david@...hat.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>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "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 v11 10/20] x86/virt/tdx: Add placeholder to construct
 TDMRs to cover all TDX memory regions

On Wed, 2023-06-07 at 08:57 -0700, Dave Hansen wrote:
> On 6/4/23 07:27, Kai Huang wrote:
> > +struct tdmr_info_list {
> > +	void *tdmrs;	/* Flexible array to hold 'tdmr_info's */
> > +	int nr_consumed_tdmrs;	/* How many 'tdmr_info's are in use */
> 
> I'm looking back here after seeing the weird cast in the next patch.
> 
> Why is this a void* instead of a _real_ type?

I followed your suggestion in v8:

https://lore.kernel.org/linux-mm/725de6e9-e468-48ef-3bae-1e8a1b7ef0f7@intel.com/

I quoted the relevant part here:

> +/* Get the TDMR from the list at the given index. */
> +static struct tdmr_info *tdmr_entry(struct tdmr_info_list *tdmr_list,
> +				    int idx)
> +{
> +	return (struct tdmr_info *)((unsigned long)tdmr_list->first_tdmr +
> +			tdmr_list->tdmr_sz * idx);
> +}

I think that's more complicated and has more casting than necessary.
This looks nicer:

	int tdmr_info_offset = tdmr_list->tdmr_sz * idx;

	return (void *)tdmr_list->first_tdmr + tdmr_info_offset;

Also, it might even be worth keeping ->first_tdmr as a void*.  It isn't
a real C array and keeping it as void* would keep anyone from doing:

	tdmr_foo = tdmr_list->first_tdmr[foo];