linux-kernel - Re: [PATCH v3 06/16] x86/virt/tdx: Improve PAMT refcounters allocation for sparse memory

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Message-ID: <7ad102d6105b6244c32e0daebcdb2ac46a5dcc68.camel@intel.com>
Date: Wed, 24 Sep 2025 06:50:08 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Edgecombe, Rick
 P" <rick.p.edgecombe@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...hat.com"
	<mingo@...hat.com>, "kas@...nel.org" <kas@...nel.org>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"Gao, Chao" <chao.gao@...el.com>, "bp@...en8.de" <bp@...en8.de>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 06/16] x86/virt/tdx: Improve PAMT refcounters
 allocation for sparse memory

On Tue, 2025-09-23 at 17:38 +0800, Binbin Wu wrote:
> > > +/*
> > > + * Allocate PAMT reference counters for the given PFN range.
> > > + *
> > > + * It consumes 2MiB for every 1TiB of physical memory.
> > > + */
> > > +static int alloc_pamt_refcount(unsigned long start_pfn, unsigned long end_pfn)
> > > +{
> > > +	unsigned long start, end;
> > > +
> > > +	start = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(start_pfn));
> > > +	end   = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(end_pfn + 1));
> > (sorry didn't notice this in last version)
> > 
> > I don't quite follow why we need "end_pfn + 1" instead of just "end_pfn"?
> > 
> > IIUC this could result in an additional 2M range being populated
> > unnecessarily when the end_pfn is 2M aligned.
> 
> IIUC, this will not happen.
> The +1 page will be converted to 4KB, and will be ignored since in
> tdx_find_pamt_refcount() the address is divided by 2M.
> 
> To handle the address unaligned to 2M, +511 should be used instead of +1?

OK. Thanks for catching.  But I still don't get why we need end_pfn + 1.

Also, when end_pfn == 511, would this result in the second refcount being
returned for the @end, while the intention should be the first refcount?

For example, assuming we have a range [0, 2M), we only need one refcount.
And the PFN range (which comes from for_each_mem_pfn_range()) would be:

    start_pfn == 0
    end_pfn   == 512

This will results in @start pointing to the first refcount and @end
pointing to the second, IIUC.

So it seems we need:

    start = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(start_pfn));
    end   = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(end_pfn) - 1));
    start = round_down(start, PAGE_SIZE);
    end   = round_up(end, PAGE_SIZE);

?