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

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Message-ID: <f080efe3-6bf4-4631-9018-2dbf546c25fb@suse.com>
Date: Tue, 2 Dec 2025 09:38:30 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "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>,
 "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>,
 "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 2.12.25 г. 0:39 ч., Edgecombe, Rick P wrote:
> On Thu, 2025-11-27 at 18:11 +0200, Nikolay Borisov wrote:
>>> +/* Number PAMT pages to be provided to TDX module per 2M region of PA */
>>> +static int tdx_dpamt_entry_pages(void)
>>> +{
>>> +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
>>> +		return 0;
>>> +
>>> +	return tdx_sysinfo.tdmr.pamt_4k_entry_size * PTRS_PER_PTE /
>>> PAGE_SIZE;
>>> +}
>>
>> Isn't this guaranteed to return 2 always as per the ABI? Can't the
>> allocation of the 2 pages be moved closer to where it's used - in
>> tdh_phymem_pamt_add which will simplify things a bit?
> 
> 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?
> 
> Another option would be to ask TDX folks to make it fixed, and then require an
> opt-in for it to be expanded later if needed. I would have to check on them on
> the reasoning for it being dynamic sized. I'm not sure if it is *that*
> complicated at this point though. Once there is more than one, the loops becomes
> tempting. And if we loop over 2 we could easily loop over n.