linux-kernel - Re: [PATCH v4 07/16] x86/virt/tdx: Add tdx_alloc/free

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Message-ID: <0274cee22d90cbfd2b26c52b864cde6dba04fc60.camel@intel.com>
Date: Tue, 2 Dec 2025 20:02:38 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Huang, Kai" <kai.huang@...el.com>, "Li,
 Xiaoyao" <xiaoyao.li@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>, "Wu, Binbin" <binbin.wu@...el.com>,
	"kas@...nel.org" <kas@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"mingo@...hat.com" <mingo@...hat.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "tglx@...utronix.de" <tglx@...utronix.de>, "Yamahata,
 Isaku" <isaku.yamahata@...el.com>, "nik.borisov@...e.com"
	<nik.borisov@...e.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 v4 07/16] x86/virt/tdx: Add tdx_alloc/free_page() helpers

On Tue, 2025-12-02 at 09:38 +0200, Nikolay Borisov wrote:
> > Yea, it could be simpler if it was always guaranteed to be 2 pages. But it
> > was
> > my understanding that it would not be a fixed size. Can you point to what
> > docs
> > makes you think that?
> 
> Looking at the PHYMEM.PAMT.ADD ABI spec the pages being added are always 
> put into pair in rdx/r8. So e.g. looking into tdh_phymem_pamt_add rcx is 
> set to a 2mb page, and subsequently we have the memcpy which simply sets 
> the rdx/r8 input argument registers, no ? Or am I misunderstanding the 
> code?

Hmm, you are totally right. The docs specify the size of the 4k entries, but
doesn't specify that Dynamic PAMT is supposed to provide larger sizes in the
other registers. A reasonable reading could assume 2 pages always, and the usage
of the other registers seems like an assumption.

Kirill, any history here?

Otherwise, we could turn tdx_dpamt_entry_pages() into a define and shrink the
stack allocated buffers. I'll see how much removing the loops helps.