linux-kernel - Re: [PATCH v11 12/20] x86/virt/tdx: Allocate and set up PAMTs for TDMRs

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Message-ID: <19b612683cf190ab2eda55f83bee554e40c2233c.camel@intel.com>
Date:   Mon, 12 Jun 2023 02:52:13 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Hansen, Dave" <dave.hansen@...el.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "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>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.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>,
        "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 12/20] x86/virt/tdx: Allocate and set up PAMTs for
 TDMRs

On Thu, 2023-06-08 at 16:43 -0700, Dave Hansen wrote:
> On 6/8/23 16:24, kirill.shutemov@...ux.intel.com wrote:
> > >  	ret = -EINVAL;
> > > +	if (ret)
> > > +		tdmrs_free_pamt_all(&tdx_tdmr_list);
> > > +	else
> > > +		pr_info("%lu KBs allocated for PAMT.\n",
> > > +				tdmrs_count_pamt_pages(&tdx_tdmr_list) * 4);
> > "* 4"? This is very cryptic. procfs uses "<< (PAGE_SHIFT - 10)" which
> > slightly less magic to me. And just make the helper that returns kilobytes
> > to begin with, if it is the only caller.
> 
> Let's look at where this data comes from:
> 
> +static unsigned long tdmrs_count_pamt_pages(struct tdmr_info_list
> *tdmr_list)
> +{
> +	unsigned long pamt_npages = 0;
> +	int i;
> +
> +	for (i = 0; i < tdmr_list->nr_consumed_tdmrs; i++) {
> +		unsigned long pfn, npages;
> +
> +		tdmr_get_pamt(tdmr_entry(tdmr_list, i), &pfn, &npages);
> +		pamt_npages += npages;
> +	}
> 
> OK, so tdmr_get_pamt() is getting it in pages.  How is it *stored*?
> 
> +static void tdmr_get_pamt(struct tdmr_info *tdmr, unsigned long *pamt_pfn,
> +			  unsigned long *pamt_npages)
> +{
> ...
> +	pamt_sz = tdmr->pamt_4k_size + tdmr->pamt_2m_size + tdmr->pamt_1g_size;
> ++	*pamt_pfn = PHYS_PFN(pamt_base);
> +	*pamt_npages = pamt_sz >> PAGE_SHIFT;
> +}
> 
> Oh, it's actually stored in bytes.  So to print it out you actually
> convert it from bytes->pages->kbytes.  Not the best.
> 
> If tdmr_get_pamt() just returned 'pamt_size_bytes', you could do one
> conversion at:
> 
> 	free_contig_range(pamt_pfn, pamt_size_bytes >> PAGE_SIZE);

I thought making tdmr_get_pamt() return pamt_pfn and pamt_npages would be more
clear that PAMTs must be in 4K granularity, but I guess it doesn't matter
anyway.

If we return bytes for PAMT size, I think we should also return physical address
instead of PFN for PAMT start?

I'll change tdmr_get_pamt() to return physical address and bytes for PAMT
location and size respectively.  Please let me know if you have any comments. 

> 
> and since tdmrs_count_pamt_pages() has only one caller you can just make
> it:  tdmrs_count_pamt_kb().  The print becomes:
> 
> 	pr_info("%lu KBs allocated for PAMT.\n",
> 		tdmrs_count_pamt_kb(&tdx_tdmr_list) * 4);
> 
> and tdmrs_count_pamt_kb() does something super fancy like:
> 
> 	return pamt_size_bytes / 1024;
> 
> which makes total complete obvious sense and needs zero explanation.

Will do.

Thanks for the feedback.