linux-kernel - Re: [RFC, PATCH 05/12] KVM: TDX: Add tdx_pamt

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Message-ID: <o74datlk3nmbc6ihxisggvxzf6r25ebnh4wt5ureud4befy7nl@l3m74n6qksqx>
Date: Thu, 8 May 2025 16:19:19 +0300
From: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Vishal Annapurve <vannapurve@...gle.com>, 
	"Huang, Kai" <kai.huang@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"seanjc@...gle.com" <seanjc@...gle.com>, "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, 
	"bp@...en8.de" <bp@...en8.de>, "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, 
	"x86@...nel.org" <x86@...nel.org>, "mingo@...hat.com" <mingo@...hat.com>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, "Yamahata, Isaku" <isaku.yamahata@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC, PATCH 05/12] KVM: TDX: Add tdx_pamt_get()/put() helpers

On Wed, May 07, 2025 at 10:42:25AM +0800, Yan Zhao wrote:
> On Tue, May 06, 2025 at 06:15:40PM -0700, Vishal Annapurve wrote:
> > On Tue, May 6, 2025 at 6:04 PM Yan Zhao <yan.y.zhao@...el.com> wrote:
> > >
> > > On Mon, May 05, 2025 at 08:44:26PM +0800, Huang, Kai wrote:
> > > > On Fri, 2025-05-02 at 16:08 +0300, Kirill A. Shutemov wrote:
> > > > > +static int tdx_pamt_add(atomic_t *pamt_refcount, unsigned long hpa,
> > > > > +                   struct list_head *pamt_pages)
> > > > > +{
> > > > > +   u64 err;
> > > > > +
> > > > > +   hpa = ALIGN_DOWN(hpa, SZ_2M);
> > > > > +
> > > > > +   spin_lock(&pamt_lock);
> > > >
> > > > Just curious, Can the lock be per-2M-range?
> > > Me too.
> > > Could we introduce smaller locks each covering a 2M range?
> > >
> > > And could we deposit 2 pamt pages per-2M hpa range no matter if it's finally
> > > mapped as a huge page or not?
> > >
> > 
> > Are you suggesting to keep 2 PAMT pages allocated for each private 2M
> > page even if it's mapped as a hugepage? It will lead to wastage of
> > memory of 4 MB per 1GB of guest memory range. For large VM sizes that
> > will amount to high values.
> Ok. I'm thinking of the possibility to aligning the time of PAMT page allocation
> to that of physical page allocation.

No. That's mostly wasted memory. We need to aim to allocate memory only as
needed. With huge pages wast majority of such allocations will never be
needed.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov