From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pg1-f202.google.com (mail-pg1-f202.google.com [209.85.215.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4F144257AFB for ; Fri, 17 Oct 2025 00:33:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.215.202 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760661183; cv=none; b=qVPM1bajndI/rLMMAPHE5N/k7YuhcqjaStK1pModMW+8sHDfrIwyZ+r97dWea8aOwtBKGurNIoIMAzD2LbFv0IVRCmIxxCjQhTbMCE5lWofaaklA3lYdcQBp6wQFgBeLCQQQBZIGelakWB9jLF3mwZv6ouAqFkjBq5sqnJOVJu0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760661183; c=relaxed/simple; bh=3mm/N8f8OCP3jzKgT6vjuBOSEtjI242afa4+FgXcNGc=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=OpZT7b7NPZm8bOxAjGX87Pl4w4RLce9M2eoXOlwrqyv9QNqSyntElUBSvJ0yOLZCzzwFwY3a/A0vD2S+oiFZW0xkNtV2TDW8i+jJh4ZPC+Aq13Wy8FDdZegM58GBZBrwy/iIeUphvqcCiPPZbWxy9ECEuiaQtEiTNsGcn7XMjuE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=JWI9apVo; arc=none smtp.client-ip=209.85.215.202 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="JWI9apVo" Received: by mail-pg1-f202.google.com with SMTP id 41be03b00d2f7-b552f91033cso1912214a12.1 for ; Thu, 16 Oct 2025 17:33:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1760661181; x=1761265981; darn=lists.linux.dev; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:from:to:cc:subject:date:message-id:reply-to; bh=cf8N8Y56G5w6bizwE7Ci0y1MCMFb0ZkjYruE6L135MI=; b=JWI9apVoJzXw1Qnc9nkf9rSHF9/FjIy7dj6nRUZEGn41YK0geDW1nzciw8vRJvM1qX wlmV70If4pkNva1n/hcLH4TFlAAEYEnNLT0pZp8Grjlnf4wAG7lgpRW1S5Ir0oMEt37N 3iLB+W7rpA2ZficyAC9JH5zbUX1zZroL68SubZIbPCbFnI33s0HtcQlfEGtVPMLGLetu 5SFPhEP+bCoZzY/EI4bZqc6j/19iPqDtcMq/qeJCgXI/cq1NJhXNUITev7rVr3sq0EwP Ski8ToTeZWYaLoFEhHJOuOsLt60qcvb9JMHUhy1Y8HR4EHUhztRMp5kVm1aonwcVDIOd AuWg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1760661181; x=1761265981; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=cf8N8Y56G5w6bizwE7Ci0y1MCMFb0ZkjYruE6L135MI=; b=SqpYDudsQNOGg5MNCP6BVsAhdu64DXHJ/YIsQTmCfpWPUXz76I4bfX5E1Pig/Z/FyL Q80jo1kYjI0SI4k9a5WTriPb0V/wOYVu3lEX56XX+Z+14zNHK6EpmEJOaPHxQiTTgUik LGrrvMd0Cb7oTl58SF0OO59KQo5qFqdOpf0uNbX4ozX/7VYME7m/5Et9KYbH3XASapPq f4PmbeXhpxi2XMF/dPGIVSxqThJcajmXEm9+9xFcGGl/HUIrI3YDq/mvz7lYwTubuyhT 9jFZ7P2CxFpr64O0MyIqefgk1YKC1oH4cH5KKrnCpB3rqjIPFKQPkyRW3+Bhw/FUt9cV CAkg== X-Forwarded-Encrypted: i=1; AJvYcCXCuuVdSmrqJO5F52FdoZDnXja3h92eUgMxcfMQPro2rzdLiKikixC6NSNuCrgsPmQkuQO5jDLcQbes@lists.linux.dev X-Gm-Message-State: AOJu0YyZphUBwQo8BMWFmdLfdF+u+4dpT1UDcaqJkRW8kARNnrQEjj0i 1x/FC2bCecpQRmyT+lMlw/kytCKBJrFVjp8hEhKYGQ3voE1plmsbItH1gD08zKmwiM4IyHqDm6S U8f9oOA== X-Google-Smtp-Source: AGHT+IFGX/O8rAylXetPrL4mM3U8URlaleqJu641GFX2gsiAqRo0BdBbaKA0U4x8ZGwd9In5RU4acPt36Xo= X-Received: from pjbml22.prod.google.com ([2002:a17:90b:3616:b0:339:ae3b:2bc7]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6300:2408:b0:334:a23e:2dff with SMTP id adf61e73a8af0-334a858f4e4mr1825671637.25.1760661180701; Thu, 16 Oct 2025 17:33:00 -0700 (PDT) Reply-To: Sean Christopherson Date: Thu, 16 Oct 2025 17:32:25 -0700 In-Reply-To: <20251017003244.186495-1-seanjc@google.com> Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20251017003244.186495-1-seanjc@google.com> X-Mailer: git-send-email 2.51.0.858.gf9c4a03a3a-goog Message-ID: <20251017003244.186495-8-seanjc@google.com> Subject: [PATCH v3 07/25] KVM: TDX: Drop superfluous page pinning in S-EPT management From: Sean Christopherson To: Marc Zyngier , Oliver Upton , Tianrui Zhao , Bibo Mao , Huacai Chen , Madhavan Srinivasan , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Sean Christopherson , Paolo Bonzini , "Kirill A. Shutemov" Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, x86@kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org, Ira Weiny , Kai Huang , Michael Roth , Yan Zhao , Vishal Annapurve , Rick Edgecombe , Ackerley Tng , Binbin Wu Content-Type: text/plain; charset="UTF-8" From: Yan Zhao Don't explicitly pin pages when mapping pages into the S-EPT, guest_memfd doesn't support page migration in any capacity, i.e. there are no migrate callbacks because guest_memfd pages *can't* be migrated. See the WARN in kvm_gmem_migrate_folio(). Eliminating TDX's explicit pinning will also enable guest_memfd to support in-place conversion between shared and private memory[1][2]. Because KVM cannot distinguish between speculative/transient refcounts and the intentional refcount for TDX on private pages[3], failing to release private page refcount in TDX could cause guest_memfd to indefinitely wait on decreasing the refcount for the splitting. Under normal conditions, not holding an extra page refcount in TDX is safe because guest_memfd ensures pages are retained until its invalidation notification to KVM MMU is completed. However, if there're bugs in KVM/TDX module, not holding an extra refcount when a page is mapped in S-EPT could result in a page being released from guest_memfd while still mapped in the S-EPT. But, doing work to make a fatal error slightly less fatal is a net negative when that extra work adds complexity and confusion. Several approaches were considered to address the refcount issue, including - Attempting to modify the KVM unmap operation to return a failure, which was deemed too complex and potentially incorrect[4]. - Increasing the folio reference count only upon S-EPT zapping failure[5]. - Use page flags or page_ext to indicate a page is still used by TDX[6], which does not work for HVO (HugeTLB Vmemmap Optimization). - Setting HWPOISON bit or leveraging folio_set_hugetlb_hwpoison()[7]. Due to the complexity or inappropriateness of these approaches, and the fact that S-EPT zapping failure is currently only possible when there are bugs in the KVM or TDX module, which is very rare in a production kernel, a straightforward approach of simply not holding the page reference count in TDX was chosen[8]. When S-EPT zapping errors occur, KVM_BUG_ON() is invoked to kick off all vCPUs and mark the VM as dead. Although there is a potential window that a private page mapped in the S-EPT could be reallocated and used outside the VM, the loud warning from KVM_BUG_ON() should provide sufficient debug information. To be robust against bugs, the user can enable panic_on_warn as normal. Link: https://lore.kernel.org/all/cover.1747264138.git.ackerleytng@google.com [1] Link: https://youtu.be/UnBKahkAon4 [2] Link: https://lore.kernel.org/all/CAGtprH_ypohFy9TOJ8Emm_roT4XbQUtLKZNFcM6Fr+fhTFkE0Q@mail.gmail.com [3] Link: https://lore.kernel.org/all/aEEEJbTzlncbRaRA@yzhao56-desk.sh.intel.com [4] Link: https://lore.kernel.org/all/aE%2Fq9VKkmaCcuwpU@yzhao56-desk.sh.intel.com [5] Link: https://lore.kernel.org/all/aFkeBtuNBN1RrDAJ@yzhao56-desk.sh.intel.com [6] Link: https://lore.kernel.org/all/diqzy0tikran.fsf@ackerleytng-ctop.c.googlers.com [7] Link: https://lore.kernel.org/all/53ea5239f8ef9d8df9af593647243c10435fd219.camel@intel.com [8] Suggested-by: Vishal Annapurve Suggested-by: Ackerley Tng Suggested-by: Rick Edgecombe Signed-off-by: Yan Zhao Reviewed-by: Ira Weiny Reviewed-by: Kai Huang [sean: extract out of hugepage series, massage changelog accordingly] Reviewed-by: Binbin Wu Signed-off-by: Sean Christopherson --- arch/x86/kvm/vmx/tdx.c | 28 ++++------------------------ 1 file changed, 4 insertions(+), 24 deletions(-) diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c index 29f344af4cc2..c3bae6b96dc4 100644 --- a/arch/x86/kvm/vmx/tdx.c +++ b/arch/x86/kvm/vmx/tdx.c @@ -1583,29 +1583,22 @@ void tdx_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa, int pgd_level) td_vmcs_write64(to_tdx(vcpu), SHARED_EPT_POINTER, root_hpa); } -static void tdx_unpin(struct kvm *kvm, struct page *page) -{ - put_page(page); -} - static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn, - enum pg_level level, struct page *page) + enum pg_level level, kvm_pfn_t pfn) { int tdx_level = pg_level_to_tdx_sept_level(level); struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm); + struct page *page = pfn_to_page(pfn); gpa_t gpa = gfn_to_gpa(gfn); u64 entry, level_state; u64 err; err = tdh_mem_page_aug(&kvm_tdx->td, gpa, tdx_level, page, &entry, &level_state); - if (unlikely(tdx_operand_busy(err))) { - tdx_unpin(kvm, page); + if (unlikely(tdx_operand_busy(err))) return -EBUSY; - } if (KVM_BUG_ON(err, kvm)) { pr_tdx_error_2(TDH_MEM_PAGE_AUG, err, entry, level_state); - tdx_unpin(kvm, page); return -EIO; } @@ -1639,29 +1632,18 @@ static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn, enum pg_level level, kvm_pfn_t pfn) { struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm); - struct page *page = pfn_to_page(pfn); /* TODO: handle large pages. */ if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm)) return -EINVAL; - /* - * Because guest_memfd doesn't support page migration with - * a_ops->migrate_folio (yet), no callback is triggered for KVM on page - * migration. Until guest_memfd supports page migration, prevent page - * migration. - * TODO: Once guest_memfd introduces callback on page migration, - * implement it and remove get_page/put_page(). - */ - get_page(page); - /* * Read 'pre_fault_allowed' before 'kvm_tdx->state'; see matching * barrier in tdx_td_finalize(). */ smp_rmb(); if (likely(kvm_tdx->state == TD_STATE_RUNNABLE)) - return tdx_mem_page_aug(kvm, gfn, level, page); + return tdx_mem_page_aug(kvm, gfn, level, pfn); return tdx_mem_page_record_premap_cnt(kvm, gfn, level, pfn); } @@ -1712,7 +1694,6 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn, return -EIO; } tdx_quirk_reset_page(page); - tdx_unpin(kvm, page); return 0; } @@ -1792,7 +1773,6 @@ static int tdx_sept_zap_private_spte(struct kvm *kvm, gfn_t gfn, if (tdx_is_sept_zap_err_due_to_premap(kvm_tdx, err, entry, level) && !KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm)) { atomic64_dec(&kvm_tdx->nr_premapped); - tdx_unpin(kvm, page); return 0; } -- 2.51.0.858.gf9c4a03a3a-goog