linux-kernel - Re: [PATCH v3 08/16] x86/virt/tdx: Optimize tdx_alloc/free

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Message-ID: <yexfu34vq5wrctlhafkrhvwfdcp5aenkleq6zolgqquz74dmzh@5336rz6kfulo>
Date: Fri, 19 Sep 2025 10:39:02 +0100
From: Kiryl Shutsemau <kas@...nel.org>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: 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, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 08/16] x86/virt/tdx: Optimize tdx_alloc/free_page()
 helpers

On Thu, Sep 18, 2025 at 04:22:16PM -0700, Rick Edgecombe wrote:
> +		} else if (IS_TDX_HPA_RANGE_NOT_FREE(tdx_status)) {
> +			/*
> +			 * Less obviously, another CPU's call to tdx_pamt_put() could have
> +			 * decremented the refcount before entering its lock section.
> +			 * In this case, the PAMT is not actually removed yet. Luckily
> +			 * TDX module tells about this case, so increment the refcount
> +			 * 0-1, so tdx_pamt_put() skips its pending PAMT.REMOVE.
> +			 *
> +			 * The call didn't need the pages though, so free them.
> +			 */
>  			atomic_inc(pamt_refcount);

It is still 0->1 transition. atomic_set(pamt_refcount, 1) would be
slightly faster here.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov