From nobody Fri Oct 3 15:31:38 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 8A367217716 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=1756426002; cv=none; b=ZjXzBqsjb861zPNwx/bgcshnnOj83AvfShBwTrPUsdkFyAZjK6gEuL9HEGW+AMfoQyGJhGM2erH7eUBUTQiQkloasr4RuDXQVQDGLJTWuGVP+rtSj+gUXjbGcKaNSRgMjQcrf08agGbZBN3e3+FQc7DWdPy2+Aaf5JZtcQ25ag0= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756426002; c=relaxed/simple; bh=gyZpzdfigGs09+ImBbJAsRRHU5xktCFn0Yw4iaqicyc=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=rAD5gstW920EieLdFGG9aCWauDH+TLd0SEtDgd0CKmLauFBjJeLDDcruymAIYifP3ajSl0vCRA5u30sCVQbKqTp/YI6mfA+W7t8XUbA0UiT97TRQggkW3cF9tNl9CHquim0GxmosR9YeGtgGOFNA/uGvbc6vF+XqCwRq9wl9FA4= 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 98e67ed59e1d1-3276575ae5eso1598871a91.1 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=k7DHZqY1jUHjX2b4jgER2GY5XrN5R7QqOKKd/XZsz9j+fk4S06dCprTOGDyaiPgYLB Oys/Y9J5wfmxAKXmSsrIlJX6v/ML9iB6uO/506MdB5Yo8itnFObhkj1aQEJiGK0d+AOZ LsfvbsKb3nHRnSnhq7hW/Nb9ArZP0zrRSBsCeTkvNHc4e8tSAQMK80VuaQdtPrnzpETN oIherFcIPVEW41TFihb+XSiYAhxgrXTOBZP7hiZSIcyjgXCorOVsmS8dRkUmPbphhk9B ZmkUsCpO/UOTJxeqwMLpm52eYCZLwbluimdTnHS1Vefep1H2XWHUNYNd1B1dRzNjIXNV g8Ug== X-Forwarded-Encrypted: i=1; AJvYcCXTFmlj7lITXbWPXuKqPQuTIqNoJaZPl7eIGEILiQoZHevyfLSk6umEO1rq2TsH2h/lHronzOKxTnkc5sU=@vger.kernel.org X-Gm-Message-State: AOJu0Yz3jtZ8NbYsf3G86RBFx5skoL7+kQmW5hSK2JCM4bjWoyjNt2B/ DRkeYA1ESZSkJdd9SzNYfpcFRl+hYPeO0EvcfjPODtl0GuX3Sghw7Dz5CmdxHrMwra7ztA7we5E 9Pq2wtg== 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: linux-kernel@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-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. Reviewed-by: Rick Edgecombe Reviewed-by: Ira Weiny Signed-off-by: Sean Christopherson Reviewed-by: Binbin Wu --- 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; =20 - atomic64_dec(&kvm_tdx->nr_premapped); return 0; } =20 @@ -3151,8 +3150,7 @@ 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 2.51.0.318.gd7df087d1a-goog