linux-kernel - Re: [PATCHv2 04/12] x86/virt/tdx: Add tdx_alloc/free

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Message-ID: <6c545c841afcd23e1b3a4fcb47573ee3a178d6e1.camel@intel.com>
Date: Tue, 16 Sep 2025 00:03:26 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>
CC: "Gao, Chao" <chao.gao@...el.com>, "bp@...en8.de" <bp@...en8.de>, "Huang,
 Kai" <kai.huang@...el.com>, "x86@...nel.org" <x86@...nel.org>,
	"mingo@...hat.com" <mingo@...hat.com>, "Zhao, Yan Y" <yan.y.zhao@...el.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: [PATCHv2 04/12] x86/virt/tdx: Add tdx_alloc/free_page() helpers

On Mon, 2025-06-09 at 22:13 +0300, Kirill A. Shutemov wrote:
> +
> +static int tdx_pamt_add(atomic_t *pamt_refcount, unsigned long hpa,
> +			struct list_head *pamt_pages)
> +{
> +	u64 err;
> +
> +	guard(spinlock)(&pamt_lock);
> +
> +	hpa = ALIGN_DOWN(hpa, PMD_SIZE);
> +
> +	/* Lost race to other tdx_pamt_add() */
> +	if (atomic_read(pamt_refcount) != 0) {
> +		atomic_inc(pamt_refcount);
> +		return 1;
> +	}
> +
> +	err = tdh_phymem_pamt_add(hpa | TDX_PS_2M, pamt_pages);
> +
> +	/*
> +	 * tdx_hpa_range_not_free() is true if current task won race
> +	 * against tdx_pamt_put().
> +	 */
> +	if (err && !tdx_hpa_range_not_free(err)) {
> +		pr_err("TDH_PHYMEM_PAMT_ADD failed: %#llx\n", err);
> +		return -EIO;
> +	}
> +
> +	atomic_set(pamt_refcount, 1);
> +
> +	if (tdx_hpa_range_not_free(err))
> +		return 1;

Hey Kirill,

I couldn't figure out how this tdx_hpa_range_not_free() check helps. We are
already inside the lock also taken by any operation that might affect PAMT
state. Can you explain more about this? Otherwise I'm going to drop it for
inability to explain.

Rick

> +
> +	return 0;
> +}