From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f202.google.com (mail-pl1-f202.google.com [209.85.214.202]) (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 B1CA71EBA04 for ; Thu, 30 Jan 2025 16:13:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.202 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738253594; cv=none; b=mZwBlrPxzXn1/luZJiztBOH1BdxbDG0+97jVG429CDskBYh/rqF1+UQNTog8MLeNNoEfDt01xcgFfzBNjlHRuIFomS+2htsb3txrSlduxPh92CV9ZIylVbKHoTxKRGN1/KFfJHn+RsBa3dqfKe0YuwtKUeu59CmTf27KiGCRv24= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738253594; c=relaxed/simple; bh=YgjLvPxUsVXB/Ck7rE6LUpuzJQUXjknahmAD8taoCkk=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=f6lTuKrH8tkj/ZI1gJmt12FLLWFR+VkzlbcZPfaeW9pxep2lftBSD51b8R1VqndUZG2xk6JtLlFBo/mIIhxhkWWTHobcT/wXqykuahxZo0PonSNdlJmePOJl4hz+4G4jID1BR63li2RldknHJppovLRyF42a/n9OKcAveyUS4aU= 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=gY6fNpLE; arc=none smtp.client-ip=209.85.214.202 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="gY6fNpLE" Received: by mail-pl1-f202.google.com with SMTP id d9443c01a7336-2165433e229so21222615ad.1 for ; Thu, 30 Jan 2025 08:13:12 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1738253592; x=1738858392; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=Go3yvVJD3fxZmgQHiBckK+umdJ8mXJIaI6+e2AHHzGM=; b=gY6fNpLEx34+2/whTrbnMBS3jZFH+LTQRtzikgUZBHzemLO7MCVUngDT2inI+ZtDEo YcZwd9oRkduj+X15P9ez0uNpE5pjhYBtAiGIH8MdO8FxX3RgUQp/KF12tDAzJh9WDpma OPhPPOL3G8ZAa3npUTYo1JSOwtqgB7abLMy8iilSONdGVVm1Ob/ctE6phwehs5oVJMQG k6FnK0E/QNabEFMSJ1ZdiP+EOedEExuOQKgD/s05tUq1z895rxxp588CJHCqOGJoMaje JbJCzfO4jerEsm4vlSmkK5vxN5m+N1/yx4S3/RX5pSsMLe4KtildXRFi/EXsmUqEp4CH lE9g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1738253592; x=1738858392; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=Go3yvVJD3fxZmgQHiBckK+umdJ8mXJIaI6+e2AHHzGM=; b=dtsbli0ldKkQTrRykFCIlhaPbUR4wJNxwbhmkWPJnIlB1EYwldX3kF++R+AKM+rfb5 KtgFWHZ6Pq2DTx/aeYNBmFJO5Md+5KZfz6Zw5tNmF9GdqxTUN53ZLI/hh40rfPICyUsZ 5YQ/CaP+tEXnXW1L0vS3IdjoNH2I2xrUKpXuJbn8EOlkpAxeEnJEqfaqeEQz+QZ4naDR r8RPBO5bFOchdlWJAxIg47olJNd9Fm6wwyszgc1k9tTTLH8IzcjYgShVh0uy/cF3jZg1 bbnhqtxBC7qhmZuLBhGAmqS0kBRA3eNg966jbuy3YBEqVCpxzA4NhqToS0IAmU1DaJJz nOKw== X-Forwarded-Encrypted: i=1; AJvYcCVNaTo7vcbK26tqoSnJy+4iNqqxNlYB4NDbspYi9Sj5mzOdRR2LJlumdKzaMhyIROVezTS25qDtAai95Bk=@vger.kernel.org X-Gm-Message-State: AOJu0YzyketlCxQ9/AJrebh5quozL6DjECZXySjl5lakj3xcjxFw4gmN UPvtTEMQB6/4IlzvDqfq8yMRoAqPSVQGlXGQVZtzeU5EaXYcE1kjeXRBqUKbRPvizntVmbabwbW yeA== X-Google-Smtp-Source: AGHT+IE9MT8MEhg+Tsk3q1TDBqwdCo25qpv8AculmFcdDqrZwNhpbu2xmSgkCqtb13dCwecV7pSI7SAxOZw= X-Received: from pfbea16.prod.google.com ([2002:a05:6a00:4c10:b0:728:aad0:33a4]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:2d27:b0:1e1:a716:3172 with SMTP id adf61e73a8af0-1ed7a4dcd4cmr11333694637.12.1738253592042; Thu, 30 Jan 2025 08:13:12 -0800 (PST) Date: Thu, 30 Jan 2025 08:13:10 -0800 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250130010825.220346-1-seanjc@google.com> Message-ID: Subject: Re: [PATCH] KVM: nSVM: Enter guest mode before initializing nested NPT MMU From: Sean Christopherson To: Yosry Ahmed Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="us-ascii" On Thu, Jan 30, 2025, Yosry Ahmed wrote: > On Wed, Jan 29, 2025 at 05:08:25PM -0800, Sean Christopherson wrote: > > When preparing vmcb02 for nested VMRUN (or state restore), "enter" guest > > mode prior to initializing the MMU for nested NPT so that guest_mode is > > set in the MMU's role. KVM's model is that all L2 MMUs are tagged with > > guest_mode, as the behavior of hypervisor MMUs tends to be significantly > > different than kernel MMUs. > > > > Practically speaking, the bug is relatively benign, as KVM only directly > > queries role.guest_mode in kvm_mmu_free_guest_mode_roots(), which SVM > > doesn't use, and in paths that are optimizations (mmu_page_zap_pte() and > > shadow_mmu_try_split_huge_pages()). > > Just curious, what about kvm_mmu_page_ad_need_write_protect()? Doh, I missed that usage. > Is it also bengin? Yes. Better to be lucky than good :-) That path forces KVM to use write-protection instead of dirty-bit based Page Modification Logging (PML) when L2 is active, because the GPAs captured by the CPU would be L2 GPAs, not L1 GPAs, and there's no guarantee that the L2=>L1 translation would be valid when KVM processes the PML buffer. To ensure the correct page gets marked dirty, KVM uses it's standard write-protect scheme when running L2, even if KVM is using PML to dirty log L1 accesses. Lucky for me, PML isn't supported on any AMD CPUs.