From nobody Fri Oct 3 19:17:25 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 F3394201113 for ; Wed, 27 Aug 2025 00:05:38 +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=1756253140; cv=none; b=mSD9v/DgdQ6xbEA3wvTwDQyzURqTsJQr17bmSRk1u/E3xtx0pD40xDAbx60CWR+WH/Qz7J2UYKAIkzRVyYVGBiWEy6IHeEmBNk2fV625jyaoWs4hwVmGJTppU3YhfI/XNFCEUw70D9Zrq/x1R1aaUTz+T3YoAHZ9kUiSzE4gR0Y= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756253140; c=relaxed/simple; bh=p5u10Bzkcywp/4GKkqpJgCa4qZjbXShDD0LksyC8ZDw=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=Mh/RqWKh7KADbuywe9rKxfs4ZYPVbKny6lXC+krblDvgHpSZQFAGnBfzwIra5M9XAAAoroZ/eBNUJKPOi8PhZ3L6nbjPMkJi+MxOpGw9ASvsrlGlTQdWbkgWkN7S3k3HQbxU2yqm+g+1pps59S+N8sMnPc43GWuHm/mlSEPcHWE= 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=cucoU5+S; 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="cucoU5+S" Received: by mail-pj1-f74.google.com with SMTP id 98e67ed59e1d1-3276575ae5bso551088a91.2 for ; Tue, 26 Aug 2025 17:05:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1756253138; x=1756857938; 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=7ErZgDOXv/jSDGCKcnbfir/yGG7t/FklW7bhCH57Uhk=; b=cucoU5+SFu/ZdcVSHz68vba8Z38GgiaIFjDjahNnLpXJ9nmDJt9cKDzv32/QtwJrqu gXyhd+lHgdUL3PDkYhrWEd8OiaRRFGwulUivuyolW79DO+nqNI59kumtm4TkLI3atfBj o+xdPVAoIZXubfGltnjgRV8qnLZnXCZXM7OFvhgYOTrNeKhdJGgNZdbrgck/29oKLcg2 4RHG9d6tt4qAUHgWYXuYHwmKZYYw3/3AjWCsh+D/YdKdzat1ODznoEZSbNRiXxWeTQRB ONs7IqvBLH2DrEtXulni3TdrL810cVGj2T7uJjTlisle3J+iL45608TkkQxU67bVQ9Im KzFw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1756253138; x=1756857938; 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=7ErZgDOXv/jSDGCKcnbfir/yGG7t/FklW7bhCH57Uhk=; b=LaLTHHUTjxhsSIL7HZWzh8DtWBRStclYDPW6aHFUPZHGhYX6zRh36H4fmfcMSNqYJd 2cypS/OtcPSJdkTj5cgQXkEtbluoaQiJUo7blgpdkQcD0yo3vEnwUxNGo6ZU8Smp7x7J f3iuKDPh2alGeWtn8117N7ah15nKtqVCsKJur5AyZ+9VUKSPKE4Zgjfh5V3kh4scJhjQ ykEosF/VFXXXXJ2x28G7sMBTY/ULM8/tevleq+4H0FlDQi2y65UcgE+5qfcCp9gOioum oqmkcj9VrCeArFgspuTY78hfNVplXi8QbtJos/Bsk5equPXJNsGUkWbp4MGpZpoaIBEK QTTw== X-Forwarded-Encrypted: i=1; AJvYcCWYrX+G89/sODesi9dfuNaZAPeLHBDI9dAZmkANRaZCFRttrUFwylf9TInxF+Gqvmao5O1nzJ/WUiZxWEU=@vger.kernel.org X-Gm-Message-State: AOJu0Yyl5hnVwZDudCCalTW9gIoELE4Qh/36JmCXzFONjDugzsCYaEj6 61PPchHeIofte0pNUHFqyncv+sStXpF2J32moRbNTDGdhwdK4Tc6cU/9ikCvZudhzq8xxWeRRs0 +o1NdIw== X-Google-Smtp-Source: AGHT+IHOit1CgdGOtEwyoWNstM/rY5EyZOJUqw3pif7druLduvP2SLwxlmjzkU+xmrbDuQykH2FtGTD0ZlE= X-Received: from pjbsp3.prod.google.com ([2002:a17:90b:52c3:b0:31f:61fc:b283]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:4b8f:b0:327:41c8:8840 with SMTP id 98e67ed59e1d1-32741c8899emr6039622a91.37.1756253138478; Tue, 26 Aug 2025 17:05:38 -0700 (PDT) Reply-To: Sean Christopherson Date: Tue, 26 Aug 2025 17:05:18 -0700 In-Reply-To: <20250827000522.4022426-1-seanjc@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250827000522.4022426-1-seanjc@google.com> X-Mailer: git-send-email 2.51.0.268.g9569e192d0-goog Message-ID: <20250827000522.4022426-9-seanjc@google.com> Subject: [RFC PATCH 08/12] KVM: TDX: Use atomic64_dec_return() instead of a poor equivalent From: Sean Christopherson To: Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Michael Roth , Yan Zhao , Ira Weiny , Vishal Annapurve , Rick Edgecombe Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Use atomic64_dec_return() when decrementing the number of "pre-mapped" S-EPT pages to ensure that the count can't go negative without KVM noticing. In theory, checking for '0' and then decrementing in a separate operation could miss a 0=3D>-1 transition. In practice, such a condition is impossible because nr_premapped is protected by slots_lock, i.e. doesn't actually need to be an atomic (that wart will be addressed shortly). Don't bother trying to keep the count non-negative, as the KVM_BUG_ON() ensures the VM is dead, i.e. there's no point in trying to limp along. Signed-off-by: Sean Christopherson Reviewed-by: Ira Weiny Reviewed-by: Rick Edgecombe --- arch/x86/kvm/vmx/tdx.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c index 88079e2d45fb..b7559ea1e353 100644 --- a/arch/x86/kvm/vmx/tdx.c +++ b/arch/x86/kvm/vmx/tdx.c @@ -1774,10 +1774,9 @@ static int tdx_sept_zap_private_spte(struct kvm *kvm= , gfn_t gfn, tdx_no_vcpus_enter_stop(kvm); } if (tdx_is_sept_zap_err_due_to_premap(kvm_tdx, err, entry, level)) { - if (KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm)) + if (KVM_BUG_ON(atomic64_dec_return(&kvm_tdx->nr_premapped) < 0, kvm)) return -EIO; =20 - atomic64_dec(&kvm_tdx->nr_premapped); return 0; } =20 @@ -3162,8 +3161,7 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gf= n_t gfn, kvm_pfn_t pfn, goto out; } =20 - if (!KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm)) - atomic64_dec(&kvm_tdx->nr_premapped); + KVM_BUG_ON(atomic64_dec_return(&kvm_tdx->nr_premapped) < 0, kvm); =20 if (arg->flags & KVM_TDX_MEASURE_MEMORY_REGION) { for (i =3D 0; i < PAGE_SIZE; i +=3D TDX_EXTENDMR_CHUNKSIZE) { --=20 2.51.0.268.g9569e192d0-goog