linux-kernel - Re: [PATCH v3 12/16] x86/virt/tdx: Add helpers to allow for pre-allocating pages

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Message-ID: <aNiSJh/65oUp+4Dr@yzhao56-desk.sh.intel.com>
Date: Sun, 28 Sep 2025 09:40:54 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "Gao, Chao" <chao.gao@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"Huang, Kai" <kai.huang@...el.com>, "kas@...nel.org" <kas@...nel.org>,
	"Annapurve, Vishal" <vannapurve@...gle.com>, "bp@...en8.de" <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 12/16] x86/virt/tdx: Add helpers to allow for
 pre-allocating pages

On Sat, Sep 27, 2025 at 06:05:12AM +0800, Edgecombe, Rick P wrote:
> On Fri, 2025-09-26 at 09:44 +0800, Yan Zhao wrote:
> > > -	return kvm_mmu_topup_memory_cache(&tdx-
> > > >mmu_external_spt_cache,
> > > -					  PT64_ROOT_MAX_LEVEL);
> > > +	/* External page tables */
> > > +	min_fault_cache_size = PT64_ROOT_MAX_LEVEL;
> > 
> > min_fault_cache_size = PT64_ROOT_MAX_LEVEL - 1?
> > We don't need to allocate page for the root page.
> 
> Why change it in this patch?
The previous size determined by TDP MMU core was just unnecessarily 1 more than
required (I guess it's due to historical reasons).

But since now the size is determined by TDX itself, I think it can be accurate.

> > > +	/* Dynamic PAMT pages (if enabled) */
> > > +	min_fault_cache_size += tdx_dpamt_entry_pages() *
> > > PT64_ROOT_MAX_LEVEL;
> > > +
> > What about commenting that it's
> > tdx_dpamt_entry_pages() * ((PT64_ROOT_MAX_LEVEL - 1) + 1) ?
> > i.e.,
> > (PT64_ROOT_MAX_LEVEL  - 1) for page table pages, and 1 for guest
> > private page.
> 
> Yes the comment could be improved. I'll enhance it. Thanks.