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

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Message-ID: <89d5876f-625b-43a6-bcad-d8caa4cbda2b@suse.com>
Date: Wed, 3 Dec 2025 15:48:57 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Kiryl Shutsemau <kas@...nel.org>,
 "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>,
 "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 3.12.25 г. 15:46 ч., Kiryl Shutsemau wrote:
> 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.
> 

Considering this, I'd opt for the simplest possible approach that works 
_now_. If in the future there are changes to the ABI let's introduce 
them incrementally when their time comes.