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 87E2F217659 for ; Fri, 29 Aug 2025 00:06:39 +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=1756426001; cv=none; b=EaJztRNIbNL3WQOmCdoHEkC1EeiRoh/HiQthynXs3Jp/0q7M5vAyDmoZG4jKtyJvblBqnlq+VuiskhUDcEVZx0HGHAlh+7dacoh2ulovQ/r1AKCAq7cqVrhn3i4djIVCbPZkQeQNlkKyL1Bkai/0loPikZNBOYOvsaIFEUua4tA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756426001; c=relaxed/simple; bh=gyZpzdfigGs09+ImBbJAsRRHU5xktCFn0Yw4iaqicyc=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=tAs+N9qEv222TzpSEVQqB3hjU2CXDNPgiHJ0mAor4Fpqblycse5UlpYueznI1gWioju3tbiLK7SBVV7GSA9tbiT6d25zV9p9q5s1GcGelCL4IA/SAQ3PN3UDu6B76mfpAYnvXM9E5R1VGuo67uYWQIISeXgWAGBe9VL4oo2ITCk= 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=iWNrXZTC; 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="iWNrXZTC" Received: by mail-pj1-f74.google.com with SMTP id 98e67ed59e1d1-3278bb34a68so1474154a91.0 for ; Thu, 28 Aug 2025 17:06:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1756425999; x=1757030799; 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=ZwIMlaifmPTtVhzroBYxRLA8wwFTYLN4Sjt92uj7DWY=; b=iWNrXZTC+uVaaFU5eSNGiMo0IzTNPcS0L0iY8hZaoiECryVm2Fb1XWas14NZKUsBEz vcmHqQJ3MegEnZaP318JkRdsKWGoSLVIxvu7DJaEpTirImAZXgdX+RRpQJ+QHgFcCdwb 7Xr/BUnezzapGgp/cwDCZ+oJ8ZFGffg+bKRh0UH4Y/whrNgi5H2q7NlI1LHNmsZApVhl VO4VbsnycnuqMgJ3TrgGkCn5vit9DAqQC4ImI5BoVxHbfg6guWV3bK9592HtOdnXz1YA GSjP3IT8g4vsbvn5asAFPvYJqbfWbDxM+aJJDZqo1XISUKLl6QXcUBb8xaY5JNqC9t11 jBBQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1756425999; x=1757030799; 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=ZwIMlaifmPTtVhzroBYxRLA8wwFTYLN4Sjt92uj7DWY=; b=sL2Sq+4zicrkE8FPkaKNWx3fD9kmPczuiCIjaKS85J05feeEvZX43RAuqXTd2beWlv ofwIsuCdFX4O0peGASSb71d82ce5/d/aQmcmwnkdyYOR145CVYBtgTfKgLmScrcDWLu5 TA2mG0KAZj6qS2766TrEIvttVDQrtROX57lvdp41jDxZc45K9vOCJEx7PQ4XMA5jMxE0 cr0kPhne1Z669M2iZc0RjUYUN4DsHRmJA+iSxUcN4rXySLBosZj2aaAW7WsAwBvIbvFp EFOOu1ihMiJ7pEscgRXVGi4Ee+1HvkiQmwGqSXN8kST/DnNCE5dKd3D55CIyvUvPiMzM stzg== X-Gm-Message-State: AOJu0Yxn3gF5x+FYDYPfTLwvX9WAy9LdfLCdEY1jRkaNZ07cljTRCszi 6Lc1Sq7Rj+5MUz0CWebv2R7ee7MH+nbONfR4b7l3adOPMJMLCZIOyVNDbVf4g26Gsvgq/ha7Ejf qOruJug== X-Google-Smtp-Source: AGHT+IF5M1lm57K1T4cESxSZmN/2EWRqfoLHHHsMaumxyGGp3Yd/18q0BTU+oBYQUA0CJrPoD1MkMyVS590= X-Received: from pjbcz5.prod.google.com ([2002:a17:90a:d445:b0:327:50fa:eff9]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:1c06:b0:327:e59d:2cc2 with SMTP id 98e67ed59e1d1-327e59d2f16mr639721a91.10.1756425998989; Thu, 28 Aug 2025 17:06:38 -0700 (PDT) Reply-To: Sean Christopherson Date: Thu, 28 Aug 2025 17:06:10 -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: <20250829000618.351013-11-seanjc@google.com> Subject: [RFC PATCH v2 10/18] 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, Ira Weiny , Kai Huang , Michael Roth , Yan Zhao , Vishal Annapurve , Rick Edgecombe , Ackerley Tng 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=>-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. Reviewed-by: Rick Edgecombe Reviewed-by: Ira Weiny Signed-off-by: Sean Christopherson --- 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 cafd618ca43c..fe0815d542e3 100644 --- a/arch/x86/kvm/vmx/tdx.c +++ b/arch/x86/kvm/vmx/tdx.c @@ -1725,10 +1725,9 @@ 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; - atomic64_dec(&kvm_tdx->nr_premapped); return 0; } @@ -3151,8 +3150,7 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, goto out; } - 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); if (arg->flags & KVM_TDX_MEASURE_MEMORY_REGION) { for (i = 0; i < PAGE_SIZE; i += TDX_EXTENDMR_CHUNKSIZE) { -- 2.51.0.318.gd7df087d1a-goog