From nobody Tue Dec 16 14:43:58 2025 Received: from mail-pj1-f74.google.com (mail-pj1-f74.google.com [209.85.216.74]) (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 2A245380853 for ; Thu, 30 Oct 2025 20:10:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.74 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761855022; cv=none; b=dYVap09OP7WecmcWcFs9a+d8ql65hzrnECWECqc+Xh/m0g71BoDn1rr94T24Xj8axBmWz03GFW03bESrR0oNq5ZxmnA8yN1Eug6WhiDc023aPWbRBC4BKvU4zes2vkUGmoSuNXZWBLzXP/6Q43sjZIuM9VZBwMLhuFRUvjr1nuA= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761855022; c=relaxed/simple; bh=2Txe47VB3K7A0LqVybEnQCSyZsk6zIqB1rLpOi1mLYk=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=G5EurktrKy4fJLbU1aPkvNfdFaUeRrnBn7MV7wLDBsR9i0iT/s1I/wZFNMuK+/G/A5pwvxshN28dCp9Av+lv6JUcceHwEmwWzLZleLpf7W3S17lrFlDTUONvbeeWF2yEuYEFvAyDf/B6gdUxuPjLz3DQFHX7SI+jlIs4R7TratI= 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=xptYbRVS; arc=none smtp.client-ip=209.85.216.74 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="xptYbRVS" Received: by mail-pj1-f74.google.com with SMTP id 98e67ed59e1d1-34029b3dbfeso1710590a91.3 for ; Thu, 30 Oct 2025 13:10:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1761855019; x=1762459819; darn=vger.kernel.org; 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=sAFIK0l3RMhPR7pmj5tjYrChp81q6H152HYKl+MHkyE=; b=xptYbRVSDX/oht3xSrV564wzdmBls/0dRwhNw60hHP0PHH5Gm9u8l4BoeGKao59aAn AEQRvSFfKbV7/kGeqIwbJWo4+tBQvr0wdc5ZjiqZip2iaAAd+s/mTVto64Six4QiPYA8 2ZJHCEiKuuQIHgS15+3tPlJ8v9ltbSUtuFqCR8tW1jQyrbgxaV9oyeTefDJsvrthymyS HxoiapA5QRlQTjyG/GyScwXOgpe4FLxSsdYA2BmceGOmItdNtqNmGQVzfLvn+MH9wG3L keCrl8RGgiWzpy1hhYOAD6HHDIvBkAuGSQbi6FhWXBD3m1Y1gWClfx6MrqRzN4HLY6GU VA4w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1761855019; x=1762459819; 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=sAFIK0l3RMhPR7pmj5tjYrChp81q6H152HYKl+MHkyE=; b=AllgeRbwWX1Y0HFYYHOxcmhqeuidFQVdS8og9k02DUaOzceVS8tIT4NQt3YobHLt9J 9OKoah32OoUMNJJTq90cpHdaEd4Ob11YPZo0v2a8hj7w+O/qO+vVABjJHZjIs34tDqM9 fgWiELg1bszLq5gM1/AiUBcHL3Y2GRAuBAkwqhywBf/MUqcNBVxANw6iJ1gxCPjQh7zO h7ZoDko7sD0fKp9CDNfcYGAP3HBI9yqQ0eeQMeKsYfwaUH+SoSdQDxkL/SrXHHwTYpE9 mcst4hziD9Dx+rOnyZV3NXRixN9BvO7Y+b5K3bby4YFFLgN9VhSzJriPzjxSnwcjcvWx 63ig== X-Forwarded-Encrypted: i=1; AJvYcCWdfIeQ93WmXdhdPXEUvOaZwvEOw8TPoU9vuD2wWQwB6upKEMAd5fbNY4U0IoN4zR8nrYeqLcf0jXM8760=@vger.kernel.org X-Gm-Message-State: AOJu0YxDnqWzk9QgG9KZS9NSFGNzDPNqtzv7cyNJ8vRuEx0hJ4zWn3TV UrRcNRixhkubpSyIrQab3n+WsCIGy8RHOPCtvrwFg9VDuyR+yi/v083GR/9M8nBHMWoVByPXcce Nrdtv/g== X-Google-Smtp-Source: AGHT+IFa06BfDwKwW2VMNxuQ8ibpI7ipYGuzW9P9Yv39A5TWMZsjHlQcJLcDxxGMugGfb86zaLcKGL4PNrU= X-Received: from pjty20.prod.google.com ([2002:a17:90a:ca94:b0:340:4910:738f]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:2250:b0:340:5c27:a096 with SMTP id 98e67ed59e1d1-34082faafd2mr1400500a91.6.1761855018548; Thu, 30 Oct 2025 13:10:18 -0700 (PDT) Reply-To: Sean Christopherson Date: Thu, 30 Oct 2025 13:09:31 -0700 In-Reply-To: <20251030200951.3402865-1-seanjc@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20251030200951.3402865-1-seanjc@google.com> X-Mailer: git-send-email 2.51.1.930.gacf6e81ea2-goog Message-ID: <20251030200951.3402865-9-seanjc@google.com> Subject: [PATCH v4 08/28] 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 , Binbin Wu , Michael Roth , Yan Zhao , Vishal Annapurve , Rick Edgecombe , Ackerley Tng Content-Transfer-Encoding: quoted-printable 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.c= om [1] Link: https://youtu.be/UnBKahkAon4 [2] Link: https://lore.kernel.org/all/CAGtprH_ypohFy9TOJ8Emm_roT4XbQUtLKZNFcM6F= r+fhTFkE0Q@mail.gmail.com [3] Link: https://lore.kernel.org/all/aEEEJbTzlncbRaRA@yzhao56-desk.sh.intel.co= m [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.co= m [6] Link: https://lore.kernel.org/all/diqzy0tikran.fsf@ackerleytng-ctop.c.googl= ers.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 Reviewed-by: Rick Edgecombe 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); } =20 -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 =3D pg_level_to_tdx_sept_level(level); struct kvm_tdx *kvm_tdx =3D to_kvm_tdx(kvm); + struct page *page =3D pfn_to_page(pfn); gpa_t gpa =3D gfn_to_gpa(gfn); u64 entry, level_state; u64 err; =20 err =3D tdh_mem_page_aug(&kvm_tdx->td, gpa, tdx_level, page, &entry, &lev= el_state); - if (unlikely(tdx_operand_busy(err))) { - tdx_unpin(kvm, page); + if (unlikely(tdx_operand_busy(err))) return -EBUSY; - } =20 if (KVM_BUG_ON(err, kvm)) { pr_tdx_error_2(TDH_MEM_PAGE_AUG, err, entry, level_state); - tdx_unpin(kvm, page); return -EIO; } =20 @@ -1639,29 +1632,18 @@ static int tdx_sept_set_private_spte(struct kvm *kv= m, gfn_t gfn, enum pg_level level, kvm_pfn_t pfn) { struct kvm_tdx *kvm_tdx =3D to_kvm_tdx(kvm); - struct page *page =3D pfn_to_page(pfn); =20 /* TODO: handle large pages. */ if (KVM_BUG_ON(level !=3D PG_LEVEL_4K, kvm)) return -EINVAL; =20 - /* - * 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 =3D=3D TD_STATE_RUNNABLE)) - return tdx_mem_page_aug(kvm, gfn, level, page); + return tdx_mem_page_aug(kvm, gfn, level, pfn); =20 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; } =20 @@ -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; } =20 --=20 2.51.1.930.gacf6e81ea2-goog