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

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Message-ID: <bfaswqmlsyycr3alibn6f422cjtpd6ybssjekvrrz4zdwgwfcz@pxy25ra4sln2>
Date: Tue, 16 Sep 2025 10:22:26 +0100
From: Kiryl Shutsemau <kirill@...temov.name>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "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>, "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 Tue, Sep 16, 2025 at 12:03:26AM +0000, Edgecombe, Rick P wrote:
> 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.

My git has comment for the check:

https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/tree/arch/x86/virt/vmx/tdx/tdx.c?h=tdx/dpamt&id=375706fe73a8499dbdddb22c13d19d7286280ad6#n2160

Consider the following scenario

		CPU0					CPU1
tdx_pamt_put()
  atomic_dec_and_test() == true
  					tdx_pamt_get()
					  atomic_inc_not_zero() == false
					  tdx_pamt_add()
					    <takes pamt_lock>
					    // CPU0 never removed PAMT memory
					    tdh_phymem_pamt_add() == HPA_RANGE_NOT_FREE
					    atomic_set(1);
					    <drops pamt_lock>
  <takes pamt_lock>
  // Lost the race to CPU1
  atomic_read() > 0
  <drop pamt_lock>

Does it make sense?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov