linux-kernel - Re: [PATCH v3 06/16] x86/virt/tdx: Improve PAMT refcounters allocation for sparse memory

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Message-ID: <28434cbae5f2eca42f783d4ad7a52206b6b85ed2.camel@intel.com>
Date: Wed, 15 Oct 2025 01:35:15 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "kas@...nel.org"
	<kas@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...hat.com"
	<mingo@...hat.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "Annapurve, Vishal" <vannapurve@...gle.com>, "Gao,
 Chao" <chao.gao@...el.com>, "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>
CC: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 06/16] x86/virt/tdx: Improve PAMT refcounters
 allocation for sparse memory


> +/*
> + * Allocate PAMT reference counters for the given PFN range.
> + *
> + * It consumes 2MiB for every 1TiB of physical memory.
> + */
> +static int alloc_pamt_refcount(unsigned long start_pfn, unsigned long end_pfn)
> +{
> +	unsigned long start, end;
> +
> +	start = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(start_pfn));
> +	end   = (unsigned long)tdx_find_pamt_refcount(PFN_PHYS(end_pfn + 1));
> +	start = round_down(start, PAGE_SIZE);
> +	end   = round_up(end, PAGE_SIZE);
> +
> +	return apply_to_page_range(&init_mm, start, end - start,
> +				   pamt_refcount_populate, NULL);
> +}
> 

This alloc_pamt_refcount() needs to check whether DPAMT is supported
before calling apply_to_page_range().  Otherwise when DPAMT is not
supported the apply_to_page_range() panics due to the @pamt_refcounts is
never initialized to a proper value.

> @@ -288,10 +377,19 @@ static int build_tdx_memlist(struct list_head *tmb_list)
>  		ret = add_tdx_memblock(tmb_list, start_pfn, end_pfn, nid);
>  		if (ret)
>  			goto err;
> +
> +		/* Allocated PAMT refcountes for the memblock */
> +		ret = alloc_pamt_refcount(start_pfn, end_pfn);
> +		if (ret)
> +			goto err;
>  	}