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

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Message-ID: <7xbqq2uplwkc36q6jyorxe6u3fboka3snwar6parado5ysz25o@qrstyzh3okgh>
Date: Wed, 3 Dec 2025 13:46:03 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: "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>, "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>, 
	"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, Dec 02, 2025 at 08:02:38PM +0000, Edgecombe, Rick P wrote:
> 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?

There was a plan to future prove DPAMT by allowing PAMT descriptor to
grow in the future. The concrete approach was not settled last time I
checked. This code was my attempt to accommodate it. I don't know if it
fits the current plan.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov