From nobody Fri Apr 3 05:49:42 2026 Received: from mgamail.intel.com (mgamail.intel.com [192.198.163.16]) (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 E682D2147E5; Fri, 3 Apr 2026 00:31:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=192.198.163.16 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775176268; cv=none; b=GjprjLp7gBuKzFCS39m0xmmRzssLm6TSion46hfFUCAlkrssG6H4MJJvu/kqeXFpsv6Eb4C1MfWhQzR0s358zLS+LuIFMsldnK2Dux8awZBW34umPDegx+W+a7UEhXbjv2E6JbmHOxnDDb8rYGSkJUTbcvu1iipjRwLnePk0ejc= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775176268; c=relaxed/simple; bh=vUaq1L4lvEMbQrUcYXAvv1Y3U3fF/UdA7tbV7tlCoj4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=AeGToIaq2QdTRIUWlnHRlFfDhThv2Vbg9xz7QhT143qOp/VMJ3Ug1X1NLSM1wJJ0AK/k/VRs16oktC7uPVrSCtSmwWOOuHBYCDjgcilqs7iw4X+Y6XtCB+H5awtMxJfZ2TA3a12TlxdgMWLr7SCmGcBVTrA7/QVUELR8Co+QMVU= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com; spf=pass smtp.mailfrom=linux.intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=fyInb4eX; arc=none smtp.client-ip=192.198.163.16 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="fyInb4eX" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1775176267; x=1806712267; h=date:from:to:cc:subject:message-id:references: mime-version:in-reply-to; bh=vUaq1L4lvEMbQrUcYXAvv1Y3U3fF/UdA7tbV7tlCoj4=; b=fyInb4eXHpFNZG9LPrxngm50YeCALc+SRttqQt2nZ7s6GbgpqxZV826v HV/TatP5K+DoR2P47E9RoSIuLOiVq2FTqsaFLnqsZVJzHVM3iDBnDCjMC YXEQkBCORo61AD6QwCrtHEXjE2AnB40H1TCNusJNQKO5nSpj869LxK7bz +Gdo6FAYnFZVKSqTlMVTJtXRqolEKt5qahTJGFfay/JYYHJT+hBGzHS84 CrBSOKxkkMlxji+/kLQnOQTj5zvkwIh66gSF6RinHGiTrbhJIyHoXT/ok DpeKFDWfi4OQl4GmQwAYfF6CYyFl4jzp5ljnNgxdVpa/aoAp3sw9IjviJ g==; X-CSE-ConnectionGUID: 253ljbtRSfe5H0MPiJdacg== X-CSE-MsgGUID: Y1v8StbKTgGtZX+0E5WCFA== X-IronPort-AV: E=McAfee;i="6800,10657,11747"; a="63794189" X-IronPort-AV: E=Sophos;i="6.23,156,1770624000"; d="scan'208";a="63794189" Received: from fmviesa002.fm.intel.com ([10.60.135.142]) by fmvoesa110.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Apr 2026 17:31:06 -0700 X-CSE-ConnectionGUID: uPLorF36RVGRWD7aqlSONQ== X-CSE-MsgGUID: EX1W2jVERpOaR2gxZWNRxg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.23,156,1770624000"; d="scan'208";a="250191143" Received: from guptapa-desk.jf.intel.com (HELO desk) ([10.165.239.46]) by fmviesa002-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Apr 2026 17:31:05 -0700 Date: Thu, 2 Apr 2026 17:31:04 -0700 From: Pawan Gupta To: x86@kernel.org, Jon Kohler , Nikolay Borisov , "H. Peter Anvin" , Josh Poimboeuf , David Kaplan , Sean Christopherson , Borislav Petkov , Dave Hansen , Peter Zijlstra , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , KP Singh , Jiri Olsa , "David S. Miller" , David Laight , Andy Lutomirski , Thomas Gleixner , Ingo Molnar , David Ahern , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , John Fastabend , Stanislav Fomichev , Hao Luo , Paolo Bonzini , Jonathan Corbet Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Asit Mallick , Tao Zhang , bpf@vger.kernel.org, netdev@vger.kernel.org, linux-doc@vger.kernel.org Subject: [PATCH v9 02/10] x86/bhi: Make clear_bhb_loop() effective on newer CPUs Message-ID: <20260402-vmscape-bhb-v9-2-94d16bc29774@linux.intel.com> X-Mailer: b4 0.15-dev References: <20260402-vmscape-bhb-v9-0-94d16bc29774@linux.intel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20260402-vmscape-bhb-v9-0-94d16bc29774@linux.intel.com> Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" As a mitigation for BHI, clear_bhb_loop() executes branches that overwrite the Branch History Buffer (BHB). On Alder Lake and newer parts this sequence is not sufficient because it doesn't clear enough entries. This was not an issue because these CPUs use the BHI_DIS_S hardware mitigation in the kernel. Now with VMSCAPE (BHI variant) it is also required to isolate branch history between guests and userspace. Since BHI_DIS_S only protects the kernel, the newer CPUs also use IBPB. A cheaper alternative to the current IBPB mitigation is clear_bhb_loop(). But it currently does not clear enough BHB entries to be effective on newer CPUs with larger BHB. At boot, dynamically set the loop count of clear_bhb_loop() such that it is effective on newer CPUs too. Use the X86_FEATURE_BHI_CTRL feature flag to select the appropriate loop count. Suggested-by: Dave Hansen Signed-off-by: Pawan Gupta --- arch/x86/entry/entry_64.S | 8 +++++--- arch/x86/include/asm/nospec-branch.h | 2 ++ arch/x86/kernel/cpu/bugs.c | 13 +++++++++++++ 3 files changed, 20 insertions(+), 3 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 3a180a36ca0e..bbd4b1c7ec04 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1536,7 +1536,9 @@ SYM_FUNC_START(clear_bhb_loop) ANNOTATE_NOENDBR push %rbp mov %rsp, %rbp - movl $5, %ecx + + movzbl bhb_seq_outer_loop(%rip), %ecx + ANNOTATE_INTRA_FUNCTION_CALL call 1f jmp 5f @@ -1556,8 +1558,8 @@ SYM_FUNC_START(clear_bhb_loop) * This should be ideally be: .skip 32 - (.Lret2 - 2f), 0xcc * but some Clang versions (e.g. 18) don't like this. */ - .skip 32 - 18, 0xcc -2: movl $5, %eax + .skip 32 - 20, 0xcc +2: movzbl bhb_seq_inner_loop(%rip), %eax 3: jmp 4f nop 4: sub $1, %eax diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/no= spec-branch.h index 70b377fcbc1c..87b83ae7c97f 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -548,6 +548,8 @@ DECLARE_PER_CPU(u64, x86_spec_ctrl_current); extern void update_spec_ctrl_cond(u64 val); extern u64 spec_ctrl_current(void); =20 +extern u8 bhb_seq_inner_loop, bhb_seq_outer_loop; + /* * With retpoline, we must use IBRS to restrict branch prediction * before calling into firmware. diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 83f51cab0b1e..2cb4a96247d8 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -2047,6 +2047,10 @@ enum bhi_mitigations { static enum bhi_mitigations bhi_mitigation __ro_after_init =3D IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI) ? BHI_MITIGATION_AUTO : BHI_MIT= IGATION_OFF; =20 +/* Default to short BHB sequence values */ +u8 bhb_seq_outer_loop __ro_after_init =3D 5; +u8 bhb_seq_inner_loop __ro_after_init =3D 5; + static int __init spectre_bhi_parse_cmdline(char *str) { if (!str) @@ -3242,6 +3246,15 @@ void __init cpu_select_mitigations(void) x86_spec_ctrl_base &=3D ~SPEC_CTRL_MITIGATIONS_MASK; } =20 + /* + * Switch to long BHB clear sequence on newer CPUs (with BHI_CTRL + * support), see Intel's BHI guidance. + */ + if (cpu_feature_enabled(X86_FEATURE_BHI_CTRL)) { + bhb_seq_outer_loop =3D 12; + bhb_seq_inner_loop =3D 7; + } + x86_arch_cap_msr =3D x86_read_arch_cap_msr(); =20 cpu_print_attack_vectors(); --=20 2.34.1