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

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Message-ID: <CAAhR5DEy5z4QsHQjvxv+uDOP=5fC47sK8NTcU5iRSgV5ReCOZQ@mail.gmail.com>
Date: Thu, 21 Aug 2025 14:53:31 -0500
From: Sagi Shahar <sagis@...gle.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>, "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, Aug 21, 2025 at 2:35 PM Edgecombe, Rick P
<rick.p.edgecombe@...el.com> wrote:
>
> 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().

My bad.