linux-kernel - Re: [PATCH v3 07/16] x86/virt/tdx: Add tdx_alloc/free

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Message-ID: <1f25ef49-fb7f-449c-be1d-71c19465219f@intel.com>
Date: Tue, 30 Sep 2025 22:03:53 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>, kas@...nel.org,
 bp@...en8.de, chao.gao@...el.com, dave.hansen@...ux.intel.com,
 isaku.yamahata@...el.com, kai.huang@...el.com, kvm@...r.kernel.org,
 linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org, mingo@...hat.com,
 pbonzini@...hat.com, seanjc@...gle.com, tglx@...utronix.de, x86@...nel.org,
 yan.y.zhao@...el.com, vannapurve@...gle.com
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 07/16] x86/virt/tdx: Add tdx_alloc/free_page() helpers

On 9/19/2025 7:22 AM, Rick Edgecombe wrote:

...

> +/* Bump PAMT refcount for the given page and allocate PAMT memory if needed */
> +int tdx_pamt_get(struct page *page)
> +{
> +	unsigned long hpa = ALIGN_DOWN(page_to_phys(page), PMD_SIZE);
> +	u64 pamt_pa_array[MAX_DPAMT_ARG_SIZE];
> +	atomic_t *pamt_refcount;
> +	u64 tdx_status;
> +	int ret;
> +
> +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> +		return 0;
> +
> +	ret = alloc_pamt_array(pamt_pa_array);
> +	if (ret)
> +		return ret;
> +
> +	pamt_refcount = tdx_find_pamt_refcount(hpa);
> +
> +	scoped_guard(spinlock, &pamt_lock) {
> +		if (atomic_read(pamt_refcount))

It's not what I expect the refcount to work (maybe I miss something 
seriously?)

My understanding/expectation is that, when refcount is not zero it needs 
to increment the refcount instead of simply return. And ...

> +			goto out_free;
> +
> +		tdx_status = tdh_phymem_pamt_add(hpa | TDX_PS_2M, pamt_pa_array);
> +
> +		if (IS_TDX_SUCCESS(tdx_status)) {
> +			atomic_inc(pamt_refcount);
> +		} else {
> +			pr_err("TDH_PHYMEM_PAMT_ADD failed: %#llx\n", tdx_status);
> +			goto out_free;
> +		}
> +	}
> +
> +	return ret;
> +out_free:
> +	free_pamt_array(pamt_pa_array);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(tdx_pamt_get);
> +
> +/*
> + * Drop PAMT refcount for the given page and free PAMT memory if it is no
> + * longer needed.
> + */
> +void tdx_pamt_put(struct page *page)
> +{
> +	unsigned long hpa = ALIGN_DOWN(page_to_phys(page), PMD_SIZE);
> +	u64 pamt_pa_array[MAX_DPAMT_ARG_SIZE];
> +	atomic_t *pamt_refcount;
> +	u64 tdx_status;
> +
> +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> +		return;
> +
> +	hpa = ALIGN_DOWN(hpa, PMD_SIZE);
> +
> +	pamt_refcount = tdx_find_pamt_refcount(hpa);
> +
> +	scoped_guard(spinlock, &pamt_lock) {
> +		if (!atomic_read(pamt_refcount))

...

when refcount > 1, decrease it.
when refcount is 1, decrease it and remove the PAMT page pair.

> +			return;
> +
> +		tdx_status = tdh_phymem_pamt_remove(hpa | TDX_PS_2M, pamt_pa_array);
> +
> +		if (IS_TDX_SUCCESS(tdx_status)) {
> +			atomic_dec(pamt_refcount);
> +		} else {
> +			pr_err("TDH_PHYMEM_PAMT_REMOVE failed: %#llx\n", tdx_status);
> +			return;
> +		}
> +	}
> +
> +	free_pamt_array(pamt_pa_array);
> +}
> +EXPORT_SYMBOL_GPL(tdx_pamt_put);