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

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Message-ID: <9be8a4d6-41d6-4e44-af98-bd2d6eae6f71@intel.com>
Date: Tue, 30 Sep 2025 10:38:30 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>,
 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 9/30/25 07:03, Xiaoyao Li wrote:
> My understanding/expectation is that, when refcount is not zero it needs
> to increment the refcount instead of simply return. And ...

That's a good point.

This doesn't properly handle the case where the PAMT exists and the
refcount is already incremented.

We need something like this with a fast path for a non-zero refcount
that doesn't take the lock:

	// When the refcount is nonzero, the PAMT is allocated already.
	// Bump it up and return.
	if (atomic_inc_not_zero(pamt_refcount))
		return;

	// No PAMT is present. Take the lock to ensure there is no race
	// to allocate it and proceed to allocate one.

	... existing code here

Without this, all of the atomic manipulation is within the lock and the
atomic_t could just be a plain int.