From mboxrd@z Thu Jan 1 00:00:00 1970 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 399341990D9 for ; Fri, 29 Aug 2025 00:06:30 +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=1756425992; cv=none; b=cU3SRirbGypyAgQiAq0/g2hTQ5zt2Phw98IBQBDR7er1zohEl0vccNZsECOAp3v8nB3MuomiflMFVes02EpG+dGkCe8loNIWSNmezwGoHSQaoFE+rgH5opGijEQekGBDe7QG6seW06GBPlP2ky1bBH5MQbw59ULoUVBcGzg8+Jg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756425992; c=relaxed/simple; bh=mJfwIk8zt4v2vwPMXySyeVbwDMhDnveaRkJ0ykfhjFE=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=j2hsQciehAJWD5P+gzfPNJMkvpPUfNnysZ+CE8vKQ2FnKxMbd1mb584comhUarJbkd6D2V8SXT5Ht26HPl/HDCDUQT4qAN4tjNn2M8uC0tweYc+Gn4NJezR+q4NMNlpvc9Rr6K6TtJbDmsIs7P0NNbN5hhg+xBcxc2SAY9J2vqA= 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=K78A8uzY; 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="K78A8uzY" Received: by mail-pj1-f74.google.com with SMTP id 98e67ed59e1d1-325e31cecd6so1350065a91.3 for ; Thu, 28 Aug 2025 17:06:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1756425990; x=1757030790; 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=wV1Wcluuc6pxP1WLIJ1v7G0uRRxI7XfMddwIt6Dax6U=; b=K78A8uzYtqvFpR8f47ABNQ2qnhkr0v3hzt8C7x731dpBd6vCDRxlzzNNuYyhA2k6xf S+qAxGH1SSArqNhF+uFvtSjPOALkP5eeF0wubW4B9eDbx3YoLeBSGrQxxaBd3ndr9SNv ESWxGmDqsdJI/socji+OgUfplCgftzi5ch7ZC4OjkqzOBsXOKOIzdgo+/4jUSMTY7ok7 fNj4u2XwVAJetY/z/J6teWVRnwITMJDgwEbhzuGN5egMEgum8U9JeCAU8N7iU7YtoGUQ r3/625ud/xjB1KJuZXNmWL8xqcoc+nQEQIPigqoxVfsiv9FMO6xL9SkBiQrQkarXf0u0 47Gw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1756425990; x=1757030790; 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=wV1Wcluuc6pxP1WLIJ1v7G0uRRxI7XfMddwIt6Dax6U=; b=qvyk0YHXHy5pNXyzvFsRepUvXs8vwsstokG52zR0IPCU1mqvbkGKdd0AHA3Sa5AuMD OWKdRMnSi0AwKiggOE+akGcqLoz6L1NnAgkGxdxxZFlqFSU1ITRU5nxjUMfG7A8uE488 xVLtRUUnlUaX0+X5TuJMiccrLl74zwZ6WYWVLhqHfpw8JGdzt7rd2vN6y7enYAJOExP9 CEmjA72hre9W6uzeOfIIwF+EBF5Xb1HHnrFdM/11Pmg7NpysI1mOMj/L/3nygSGrhJ+Y Oj55E1fZfrNEASgCFeLryZTvTmZpoLAn9eD1UV1o9IkkZ4kRK+kBhlcVf6FLGV/d2qLS cH9g== X-Gm-Message-State: AOJu0YwJ5oprF5uuOX9TD6D5ccMFL4pIZsPR5EYj+pR2tTKKxQjtkU+2 fPBuyEOBaUHZc27ayDAtjNzcrtKUb6/Egn8A1wSZsByBHBvr/Cvn6ihf/bX1Bm10njbCVBCPZ7w mep/+bg== X-Google-Smtp-Source: AGHT+IGfNSHJC1tKpnIi9nf7ERhW+CnNYjAM/RgFZvKWfrFvpVxfSo19+e7Hp391VLd1R2FN0hILyDkjQJU= X-Received: from pjyp15.prod.google.com ([2002:a17:90a:e70f:b0:312:e266:f849]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:384c:b0:31e:d9f0:9b96 with SMTP id 98e67ed59e1d1-32515ef8acbmr32399421a91.14.1756425990471; Thu, 28 Aug 2025 17:06:30 -0700 (PDT) Reply-To: Sean Christopherson Date: Thu, 28 Aug 2025 17:06:05 -0700 In-Reply-To: <20250829000618.351013-1-seanjc@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250829000618.351013-1-seanjc@google.com> X-Mailer: git-send-email 2.51.0.318.gd7df087d1a-goog Message-ID: <20250829000618.351013-6-seanjc@google.com> Subject: [RFC PATCH v2 05/18] KVM: TDX: Drop superfluous page pinning in S-EPT management From: Sean Christopherson To: Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Ira Weiny , Kai Huang , Michael Roth , Yan Zhao , Vishal Annapurve , Rick Edgecombe , Ackerley Tng 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] 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 c83e1ff02827..f24f8635b433 100644 --- a/arch/x86/kvm/vmx/tdx.c +++ b/arch/x86/kvm/vmx/tdx.c @@ -1586,29 +1586,22 @@ 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; } @@ -1642,29 +1635,18 @@ 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); } @@ -1715,7 +1697,6 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn, return -EIO; } tdx_clear_page(page); - tdx_unpin(kvm, page); return 0; } @@ -1795,7 +1776,6 @@ 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; } -- 2.51.0.318.gd7df087d1a-goog