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Mon, 30 Mar 2026 08:28:46 +0000 Date: Mon, 30 Mar 2026 15:49:11 +0800 From: Yan Zhao To: Rick Edgecombe CC: , , , , , , , Subject: Re: [PATCH 17/17] KVM: TDX: Move external page table freeing to TDX code Message-ID: Reply-To: Yan Zhao References: <20260327201421.2824383-1-rick.p.edgecombe@intel.com> <20260327201421.2824383-18-rick.p.edgecombe@intel.com> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260327201421.2824383-18-rick.p.edgecombe@intel.com> X-ClientProxiedBy: KUZPR01CA0019.apcprd01.prod.exchangelabs.com (2603:1096:d10:26::15) To PH0PR11MB7472.namprd11.prod.outlook.com (2603:10b6:510:28c::12) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH0PR11MB7472:EE_|MW4PR11MB7008:EE_ X-MS-Office365-Filtering-Correlation-Id: 96b3e892-0cf7-44e5-3f17-08de8e365c48 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|366016|18002099003|22082099003|56012099003; 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Page > tables needs to be freed via RCU to prevent walking one that gets > freed. > > While none of these lockless walk operations actually happen for the > mirror EPT, the TDP MMU none-the-less frees the mirror EPT page tables in > the same way, and because it’s a handy place to plug it in, the external > page tables as well. > > However, the external page tables definitely can’t be walked once they are > reclaimed from the TDX module. The TDX module releases the page for the > host VMM to use, so this RCU-time free is unnecessary for external page > tables. > > So move the free_page() call to TDX code. Create an > tdp_mmu_free_unused_sp() to allow for freeing external page tables that > have never left the TDP MMU code (i.e. don’t need freed in a special way. > > Link: https://lore.kernel.org/kvm/aYpjNrtGmogNzqwT@google.com/ > Not-yet-Signed-off-by: Sean Christopherson > [Based on a diff by Sean, added log] > Signed-off-by: Rick Edgecombe > --- > arch/x86/kvm/mmu/tdp_mmu.c | 16 +++++++++++----- > arch/x86/kvm/vmx/tdx.c | 11 ++++++++++- > 2 files changed, 21 insertions(+), 6 deletions(-) > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c > index 575033cc7fe4..18e11c1c7631 100644 > --- a/arch/x86/kvm/mmu/tdp_mmu.c > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > @@ -53,13 +53,18 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm) > rcu_barrier(); > } > > -static void tdp_mmu_free_sp(struct kvm_mmu_page *sp) > +static void __tdp_mmu_free_sp(struct kvm_mmu_page *sp) > { > - free_page((unsigned long)sp->external_spt); > free_page((unsigned long)sp->spt); > kmem_cache_free(mmu_page_header_cache, sp); > } > > +static void tdp_mmu_free_unused_sp(struct kvm_mmu_page *sp) > +{ > + free_page((unsigned long)sp->external_spt); > + __tdp_mmu_free_sp(sp); > +} > + > /* > * This is called through call_rcu in order to free TDP page table memory > * safely with respect to other kernel threads that may be operating on > @@ -73,7 +78,8 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head) > struct kvm_mmu_page *sp = container_of(head, struct kvm_mmu_page, > rcu_head); > > - tdp_mmu_free_sp(sp); > + WARN_ON_ONCE(sp->external_spt); > + __tdp_mmu_free_sp(sp); > } > > void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root) > @@ -1261,7 +1267,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) > * failed, e.g. because a different task modified the SPTE. > */ > if (r) { > - tdp_mmu_free_sp(sp); > + tdp_mmu_free_unused_sp(sp); > goto retry; > } > > @@ -1571,7 +1577,7 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm, > * installs its own sp in place of the last sp we tried to split. > */ > if (sp) > - tdp_mmu_free_sp(sp); > + tdp_mmu_free_unused_sp(sp); > > return 0; > } > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c > index d064b40a6b31..1346e891ca94 100644 > --- a/arch/x86/kvm/vmx/tdx.c > +++ b/arch/x86/kvm/vmx/tdx.c > @@ -1782,7 +1782,16 @@ static void tdx_sept_free_private_spt(struct kvm *kvm, gfn_t gfn, > */ > if (KVM_BUG_ON(is_hkid_assigned(to_kvm_tdx(kvm)), kvm) || > tdx_reclaim_page(virt_to_page(sp->external_spt))) > - sp->external_spt = NULL; > + goto out; > + > + /* > + * Immediately free the S-EPT page as the TDX subsystem doesn't support > + * freeing pages from RCU callbacks, and more importantly because > + * TDH.PHYMEM.PAGE.RECLAIM ensures there are no outstanding readers. > + */ > + free_page((unsigned long)sp->external_spt); Since sp->external_spt is allocated in kvm_mmu_alloc_external_spt() in a similar way as sp->spt in tdp_mmu_alloc_sp(), why bother making freeing of sp->external_spt special? i.e., what's the downside of freeing it together with sp->spt in tdp_mmu_free_sp() ? > +out: > + sp->external_spt = NULL; > } > > static int tdx_sept_remove_private_spte(struct kvm *kvm, gfn_t gfn, > -- > 2.53.0 >