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

[Huang, Kai]

On Wed, 2025-05-14 at 08:21 -0700, Vishal Annapurve wrote:
> On Tue, May 13, 2025 at 6:12 PM Huang, Kai <kai.huang@xxxxxxxxx> 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.

Agreed.  Thanks.

I still think all control pages and secure EPT pages can just use
tdx_{alloc|free}_page() though (because we always alloc and use them in 4K
granularity).