linux-kernel - Re: [RFC, PATCH 11/12] KVM: TDX: Reclaim PAMT memory

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Message-ID: <CAGtprH--e6i6b9grOLTUwYXKSNb=Ws5sNPniY+oJpyctM1cdTA@mail.gmail.com>
Date: Wed, 14 May 2025 08:21:32 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, pbonzini@...hat.com, seanjc@...gle.com, 
	rick.p.edgecombe@...el.com, isaku.yamahata@...el.com, yan.y.zhao@...el.com, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, kvm@...r.kernel.org, x86@...nel.org, 
	linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [RFC, PATCH 11/12] KVM: TDX: Reclaim PAMT memory

On Tue, May 13, 2025 at 6:12 PM Huang, Kai <kai.huang@...el.com> wrote:
>
>
>
> On 3/05/2025 1:08 am, Kirill A. Shutemov wrote:
> > The PAMT memory holds metadata for TDX-protected memory. With Dynamic
> > PAMT, PAMT_4K is allocated on demand. The kernel supplies the TDX module
> > with a few pages that cover 2M of host physical memory.
> >
> > PAMT memory can be reclaimed when the last user is gone. It can happen
> > in a few code paths:
> >
> > - On TDH.PHYMEM.PAGE.RECLAIM in tdx_reclaim_td_control_pages() and
> >    tdx_reclaim_page().
> >
> > - On TDH.MEM.PAGE.REMOVE in tdx_sept_drop_private_spte().
> >
> > - In tdx_sept_zap_private_spte() for pages that were in the queue to be
> >    added with TDH.MEM.PAGE.ADD, but it never happened due to an error.
> >
> > Add tdx_pamt_put() in these code paths.
>
> IMHO, instead of explicitly hooking tdx_pamt_put() to various places, we
> should just do tdx_free_page() for the pages that were allocated by
> tdx_alloc_page() (i.e., control pages, SEPT pages).
>
> That means, IMHO, we should do PAMT allocation/free when we actually
> *allocate* and *free* the target TDX private page(s).  I.e., we should:

I think it's important to ensure that PAMT pages are *only* allocated
for a 2M range if it's getting mapped in EPT at 4K granularity.
Physical memory allocation order can be different from the EPT mapping
granularity.