linux-kernel - Re: [PATCHv2 08/12] KVM: TDX: Handle PAMT allocation in fault path

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Message-ID: <0a1cf08ba04b026a6c48a390a756dc2a990b3620.camel@intel.com>
Date: Thu, 21 Aug 2025 19:35:21 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"sagis@...gle.com" <sagis@...gle.com>
CC: "Gao, Chao" <chao.gao@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"Huang, Kai" <kai.huang@...el.com>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "bp@...en8.de" <bp@...en8.de>,
	"mingo@...hat.com" <mingo@...hat.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "x86@...nel.org"
	<x86@...nel.org>, "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>
Subject: Re: [PATCHv2 08/12] KVM: TDX: Handle PAMT allocation in fault path

On Thu, 2025-08-21 at 14:21 -0500, Sagi Shahar wrote:
> >   int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
> >                                enum pg_level level, kvm_pfn_t pfn)
> >   {
> > +       struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
> >          struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> >          struct page *page = pfn_to_page(pfn);
> > +       int ret;
> > +
> > +       ret = tdx_pamt_get(page, level, tdx_alloc_pamt_page_atomic, vcpu);
> > +       if (ret)
> > +               return ret;
> 
> tdx_pamt_get() can return non-zero value in case of success e.g.
> returning 1 in case tdx_pamt_add() lost the race. 

No?

+static int tdx_pamt_get(struct page *page, enum pg_level level)
+{
+	unsigned long hpa = page_to_phys(page);
+	atomic_t *pamt_refcount;
+	LIST_HEAD(pamt_pages);
+	int ret;
+
+	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
+		return 0;
+
+	if (level != PG_LEVEL_4K)
+		return 0;
+
+	pamt_refcount = tdx_get_pamt_refcount(hpa);
+	WARN_ON_ONCE(atomic_read(pamt_refcount) < 0);
+
+	if (atomic_inc_not_zero(pamt_refcount))
+		return 0;
+
+	if (tdx_alloc_pamt_pages(&pamt_pages))
+		return -ENOMEM;
+
+	ret = tdx_pamt_add(pamt_refcount, hpa, &pamt_pages);
+	if (ret)
+		tdx_free_pamt_pages(&pamt_pages);
+
+	return ret >= 0 ? 0 : ret;
+}

> Shouldn't we check
> for (ret < 0) here and below cases?

I think you are thinking of tdx_pamt_add().