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

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Message-ID: <CAGtprH9GvBd0QLksKGan0V-RPsbJVPrsZ9PE=PPgHx11x4z1aA@mail.gmail.com>
Date: Tue, 6 May 2025 18:15:40 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: "Huang, Kai" <kai.huang@...el.com>, 
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.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 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.