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

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Message-ID: <c0590dca-13a6-4173-9b82-3604d26ce0c7@intel.com>
Date: Mon, 29 Sep 2025 11:08:17 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...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>,
 "kas@...nel.org" <kas@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
 "mingo@...hat.com" <mingo@...hat.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "tglx@...utronix.de" <tglx@...utronix.de>,
 "Yamahata, Isaku" <isaku.yamahata@...el.com>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>,
 "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
 "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 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?