linux-kernel - Re: [PATCHv2 00/12] TDX: Enable Dynamic PAMT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <z6itjtzwv6pelozn6f2kp6k4s5baeodjlptrsccbvipbfzifuh@4opoeluu6anb>
Date: Wed, 13 Aug 2025 08:49:59 +0100
From: Kiryl Shutsemau <kas@...nel.org>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Annapurve, Vishal" <vannapurve@...gle.com>, 
	"Gao, Chao" <chao.gao@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>, 
	"x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>, 
	"Huang, Kai" <kai.huang@...el.com>, "mingo@...hat.com" <mingo@...hat.com>, 
	"Zhao, Yan Y" <yan.y.zhao@...el.com>, "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCHv2 00/12] TDX: Enable Dynamic PAMT

On Tue, Aug 12, 2025 at 03:12:52PM +0000, Edgecombe, Rick P wrote:
> On Tue, 2025-08-12 at 09:04 +0100, kas@...nel.org wrote:
> > > > E.g. for things like TDCS pages and to some extent non-leaf S-EPT pages,
> > > > on-demand
> > > > PAMT management seems reasonable.  But for PAMTs that are used to track
> > > > guest-assigned
> > > > memory, which is the vaaast majority of PAMT memory, why not hook
> > > > guest_memfd?
> > > 
> > > This seems fine for 4K page backing. But when TDX VMs have huge page
> > > backing, the vast majority of private memory memory wouldn't need PAMT
> > > allocation for 4K granularity.
> > > 
> > > IIUC guest_memfd allocation happening at 2M granularity doesn't
> > > necessarily translate to 2M mapping in guest EPT entries. If the DPAMT
> > > support is to be properly utilized for huge page backings, there is a
> > > value in not attaching PAMT allocation with guest_memfd allocation.
> > 
> > Right.
> > 
> > It also requires special handling in many places in core-mm. Like, what
> > happens if THP in guest memfd got split. Who would allocate PAMT for it?
> > Migration will be more complicated too (when we get there).
> 
> I actually went down this path too, but the problem I hit was that TDX module
> wants the PAMT page size to match the S-EPT page size. And the S-EPT size will
> recall.

With DPAMT, when you pass page pair to PAMT.ADD they will be stored in the
PAMT_2M entry. So PAMT_2M entry cannot be used as a leaf entry anymore.

In theory, TDX module could stash them somewhere else, like generic memory
pool to be used for PAMT_4K when needed. But it is significantly different
design to what we have now with different set of problems.

-- 
Kiryl Shutsemau / Kirill A. Shutemov