linux-kernel - Re: [PATCH v12 08/22] x86/virt/tdx: Get information about TDX module and TDX-capable memory

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Date:   Tue, 27 Jun 2023 10:45:33 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "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>,
        "x86@...nel.org" <x86@...nel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v12 08/22] x86/virt/tdx: Get information about TDX module
 and TDX-capable memory

On Tue, 2023-06-27 at 12:51 +0300, kirill.shutemov@...ux.intel.com wrote:
> On Tue, Jun 27, 2023 at 02:12:38AM +1200, Kai Huang wrote:
> >  static int init_tdx_module(void)
> >  {
> > +	struct tdsysinfo_struct *sysinfo;
> > +	struct cmr_info *cmr_array;
> > +	int ret;
> > +
> > +	/*
> > +	 * Get the TDSYSINFO_STRUCT and CMRs from the TDX module.
> > +	 *
> > +	 * The buffers of the TDSYSINFO_STRUCT and the CMR array passed
> > +	 * to the TDX module must be 1024-bytes and 512-bytes aligned
> > +	 * respectively.  Allocate one page to accommodate them both and
> > +	 * also meet those alignment requirements.
> > +	 */
> > +	sysinfo = (struct tdsysinfo_struct *)__get_free_page(GFP_KERNEL);
> > +	if (!sysinfo)
> > +		return -ENOMEM;
> > +	cmr_array = (struct cmr_info *)((unsigned long)sysinfo + PAGE_SIZE / 2);
> > +
> > +	BUILD_BUG_ON(PAGE_SIZE / 2 < TDSYSINFO_STRUCT_SIZE);
> > +	BUILD_BUG_ON(PAGE_SIZE / 2 < sizeof(struct cmr_info) * MAX_CMRS);
> 
> This works, but why not just use slab for this? kmalloc has 512 and 1024
> pools already and you won't waste memory for rounding up.
> 
> Something like this:
> 
>         sysinfo = kmalloc(TDSYSINFO_STRUCT_SIZE, GFP_KERNEL);
>         if (!sysinfo)
>                 return -ENOMEM;
> 
>         cmr_array_size = sizeof(struct cmr_info) * MAX_CMRS;
> 
>         /* CMR array has to be 512-aligned */
>         cmr_array_size = round_up(cmr_array_size, 512);

Should we define a macro for 512

	+#define CMR_INFO_ARRAY_ALIGNMENT	512

And get rid of this comment?  AFAICT Dave didn't like such comment mentioning
512-bytes aligned if we have a macro for that.

> 
>         cmr_array = kmalloc(cmr_array_size, GFP_KERNEL);
>         if (!cmr_array) {
>                 kfree(sysinfo);
>                 return -ENOMEM;
>         }
> 
> ?
> 

I confess the reason I used __get_free_page() was to avoid having to allocate
twice, and in case of failure, I need to handle additional memory free.  But I
can do if you think it's clearer?

I wouldn't worry about wasting memory.  The buffer is freed anyway for now. 
Long-termly it's just 4K.