From nobody Fri Dec 19 04:54:13 2025 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5C5787DA8C for ; Sun, 22 Dec 2024 17:07:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734887234; cv=none; b=SV1Wk8mXNq7Vc5YWgmaFpcPl9JpYMyNQsXbXSboSjEPNAlfgYcMV7ApiocCilkZ2uTT1YCHBAiyO9GzJOH9Zni2bl9G/lcV6KbNGPTBycVxJ6VP6uTkXthDHdmHIPRFCS/g4IEPFCOkEySmcQ/jlfRUVZd/Ur5WbbB4N1vDVZVY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734887234; c=relaxed/simple; bh=IQ4+JSDMinTy+psTjBLDEErWCRdCZfAmVg/qfbOHaVY=; h=From:To:Subject:Date:Message-ID:MIME-Version:Content-Type; b=ZpkPUjb6IeyPxYK/yRSjhto7FkrBm31/rksV0Kg30XIVpPeaEpx0tfrF7fLpCCs1UV+nlUYbwxAg5h1FwIu9s71pfVU1ccRhAgGYnh6xOE5jBARqY2UDJ8G76oMnPThj8PafoCtI+AGwg3oB9qhTzwJGhJC/3ncAyDjwoZDojtw= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=dobx8Mff; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="dobx8Mff" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1734887231; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=y+R7BjqhyjEJzqinMfiFvEJo2vYtdPLc/QEalTKd9X4=; b=dobx8MffMscjU7Gh5c6E5aJ6kkL1AH0gryBrxvxnT+gDgdJKM13omfgCwZvukBxRVTH+VB ysBCUH2UyL7xCAcazSxip6jTv2qO/OmjQ7Yg0OlIanGR8JJO3mxIPzhrxH4vq7YPejnt2n /KiVfyvxBQWdJwDnlQ5BWC/ONjsTDf8= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-333-359qiiwqPE-28EftS7u0Kg-1; Sun, 22 Dec 2024 12:07:06 -0500 X-MC-Unique: 359qiiwqPE-28EftS7u0Kg-1 X-Mimecast-MFC-AGG-ID: 359qiiwqPE-28EftS7u0Kg Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id DBC8619560A2; Sun, 22 Dec 2024 17:07:05 +0000 (UTC) Received: from virtlab1023.lab.eng.rdu2.redhat.com (virtlab1023.lab.eng.rdu2.redhat.com [10.8.1.187]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 5B45319560A3; Sun, 22 Dec 2024 17:07:05 +0000 (UTC) From: Paolo Bonzini To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: [PATCH v2] KVM: x86: let it be known that ignore_msrs is a bad idea Date: Sun, 22 Dec 2024 12:07:04 -0500 Message-ID: <20241222170704.331776-1-pbonzini@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Content-Type: text/plain; charset="utf-8" When running KVM with ignore_msrs=3D1 and report_ignored_msrs=3D0, the user= has no clue that that the guest is being lied to. This may cause bug reports such as https://gitlab.com/qemu-project/qemu/-/issues/2571, where enabling a CPUID bit in QEMU caused Linux guests to try reading MSR_CU_DEF_ERR; and being lied about the existence of MSR_CU_DEF_ERR caused the guest to assume other things about the local APIC which were not true: Sep 14 12:02:53 kernel: mce: [Firmware Bug]: Your BIOS is not setting up = LVT offset 0x2 for deferred error IRQs correctly. Sep 14 12:02:53 kernel: unchecked MSR access error: RDMSR from 0x852 at r= IP: 0xffffffffb548ffa7 (native_read_msr+0x7/0x40) Sep 14 12:02:53 kernel: Call Trace: ... Sep 14 12:02:53 kernel: native_apic_msr_read+0x20/0x30 Sep 14 12:02:53 kernel: setup_APIC_eilvt+0x47/0x110 Sep 14 12:02:53 kernel: mce_amd_feature_init+0x485/0x4e0 ... Sep 14 12:02:53 kernel: [Firmware Bug]: cpu 0, try to use APIC520 (LVT of= fset 2) for vector 0xf4, but the register is already in use for vector 0x0 = on this cpu Without reported_ignored_msrs=3D0 at least the host kernel log will contain enough information to avoid going on a wild goose chase. But if reports about individual MSR accesses are being silenced too, at least complain loudly the first time a VM is started. Signed-off-by: Paolo Bonzini --- arch/x86/kvm/x86.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index c8160baf3838..12fa68a06966 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -12724,6 +12724,13 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned lon= g type) kvm_hv_init_vm(kvm); kvm_xen_init_vm(kvm); =20 + if (ignore_msrs && !report_ignored_msrs) { + pr_warn_once("Running KVM with ignore_msrs=3D1 and report_ignored_msrs= =3D0 is not a\n" + "a supported configuration. Lying to the guest about the existenc= e of MSRs\n" + "may cause the guest operating system to hang or produce errors. = If a guest\n" + "does not run without ignore_msrs=3D1, please report it to kvm@vge= r.kernel.org.\n"); + } + return 0; =20 out_uninit_mmu: --=20 2.43.5