linux-kernel - Re: [PATCH v3 05/16] x86/virt/tdx: Allocate reference counters for PAMT memory

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Message-ID: <4e1a40459724f1d4169cf962acccbe471826b7e8.camel@intel.com>
Date: Tue, 30 Sep 2025 01:04:45 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Hansen, Dave" <dave.hansen@...el.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Huang, Kai" <kai.huang@...el.com>, "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>, "pbonzini@...hat.com"
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 Isaku" <isaku.yamahata@...el.com>, "kas@...nel.org" <kas@...nel.org>,
	"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 05/16] x86/virt/tdx: Allocate reference counters for
 PAMT memory

On Mon, 2025-09-29 at 11:08 -0700, Dave Hansen wrote:
> On 9/29/25 10:41, Edgecombe, Rick P wrote:
> > On Tue, 2025-09-23 at 15:45 +0800, Binbin Wu wrote:
> > > > +/*
> > > > + * Allocate PAMT reference counters for all physical memory.
> > > > + *
> > > > + * It consumes 2MiB for every 1TiB of physical memory.
> > > > + */
> > > > +static int init_pamt_metadata(void)
> > > > +{
> > > > +	size_t size = max_pfn / PTRS_PER_PTE * sizeof(*pamt_refcounts);
> > > Is there guarantee that max_pfn is PTRS_PER_PTE aligned?
> > > If not, it should be rounded up.
> > Vmalloc() should handle it?
> 
> vmalloc() will, for instance, round up to 2 pages if you ask for 4097
> bytes in 'size'. But that's not the problem. The 'size' calculation
> itself is the problem.
> 
> You need exactly 2 MiB for every 1 TiB of memory, so let's say we have:
> 
> 	max_pfn = 1<<28
> 
> (where 28 == 40-PAGE_SIZE) then size would be *exactly* 1<<21 (2 MiB).
> Right?
> 
> But what if:
> 
> 	max_pfn = (1<<28) + 1
> 
> Then size needs to be one more page. Right? But what would the code do?

Doh, right. There is an additional issue. A later patch tweaks it to be:
+	size = max_pfn / PTRS_PER_PTE * sizeof(*pamt_refcounts);
+	size = round_up(size, PAGE_SIZE);

Perhaps an attempt to fix up the issue by Kirill? It should be fixed like Binbin
suggests, maybe:
+	size = DIV_ROUND_UP(max_pfn, PTRS_PER_PTE) * sizeof(*pamt_refcounts);

Thanks, and sorry for not giving the comment the proper attention the first time
around.