From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wm1-f54.google.com (mail-wm1-f54.google.com [209.85.128.54]) (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 AC1F235DA46 for ; Wed, 25 Mar 2026 19:41:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.54 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774467714; cv=none; b=MOhEH+UIvulqkcEod5uiMxv4RoU5Lkhkxl+dhos/3CVEdCjWmzJQl6RvWXGt5UcrIHzjUnvP8vMAAWDtMgZMi5a/7jvry4D+RbF045r3I8VZl4LQMpXvcfTnHHJ+CrY7RZbzJqXSGmg4g4M4Cooo6mnUkmzRdxnNnPGfPuHK+JU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774467714; c=relaxed/simple; bh=d+nfjMfEVHf7jtz1MIQh+squ9QrgehK0qZ/mTLc/EZw=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=tueWMPpC660+DuH3/K3tPBgXs/eSJJdT+rjXfHTy36V1usiBDFlVLLH0buqQw09KrawgqxMOMVqfpskBSVxFp9N59kQ/wSziYbicnfBlDDT/XwQeq7BegvBu4TxzzDy8a7FWptMaNvr/srAzT5IapSNzuEANgj5qWgDPeUBXwjQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=D8Zm4OEg; arc=none smtp.client-ip=209.85.128.54 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="D8Zm4OEg" Received: by mail-wm1-f54.google.com with SMTP id 5b1f17b1804b1-4852afd42ceso1834215e9.2 for ; Wed, 25 Mar 2026 12:41:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1774467711; x=1775072511; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:subject:cc:to:from:date:from:to:cc:subject:date :message-id:reply-to; bh=5agou4BHmM67c6SSsDwlj7qtFxZ2/CyU4qlcqH2Z8DI=; b=D8Zm4OEgE7zuhMoO+TLA6zFkoCvrIu6Zulcl82z1eiQnUVxOyFpLzEDJ237N96a63L q+ghLzhGjUVdtJnfPtk7wsAP1ykwBAmNJlRKmggQNUTHhMPdcqaYhrP7YYTLu7QwPU7E GaN3y5XNr27D8BOGXzhOvOU8idIz+YpoSyZfo7jO4SjvekAgsJzLKIjJWXMm0/q4xdtC R6M7fMSu/bc3U1VVmsE0QnTRmki7frlt9B+QlFeLeRPkftJTygbzwIyQ0sCjLn/MWkI9 LIcPpOgUvglVrXHmspUF2S0XPWXnf0yu54sjnr0l2jWGbRVq3qgGH7hrpA16cU+ymnpf r9mA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1774467711; x=1775072511; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from :to:cc:subject:date:message-id:reply-to; bh=5agou4BHmM67c6SSsDwlj7qtFxZ2/CyU4qlcqH2Z8DI=; b=VyAivBRBQVm01HkIVNLc837/MIitSZGcmFThxr89g/RwkO04GzhLobXRMxBpo3H+TF bQ1PWxz5snPJBGBKsP4hV21tBrgJ6iHWjN3wfgmrte3w6s8K85CUTPQOeHCe8DH1FnwV 0rFjrwgfY8FQFGLwA7B6Kzzcs/FDOIfpbo7BQKCAK/X1Ob9o+GvAdbockQheckOYVtqw DKKv3ekSWjG/SC1yHw/Ip9GJ41BV8ct91ZCJFT5PuwgnxqBh2TpwHGxa7HvTDvEOrxaE dOQMPzOmJP6TYBijzzj+A1X7vGGPa1EgMee1J4U9hqm+FVmklG/oDwWFTtI/zP7lKbrO +HZg== X-Forwarded-Encrypted: i=1; AJvYcCW5EVwAlxzUTXvLADl6taBPitj+gd0Okojlb5FxiXAT7pEowW54j+6XgaawblVmXVRoLS0=@vger.kernel.org X-Gm-Message-State: AOJu0YykMa8r1wk8Cr4HqKf11GP8C1ne2siw7o5GyRYENGrqYIUWbl+o Psl6FCKVMRJDDlojMweUBbV9AuAEZkn9Sa1MUYWrugbqazPVNRcJHmdn X-Gm-Gg: ATEYQzwa6dru9idluwHjGKBLF8KPgcC8IJbvQRnKBzE7n0gda6/sEjnS8j1lPi4RtVo o1pSg8o4ahHE8Nza64OlXu9arjC0lCyH/OojEyWB5I5sMITNDynLRACf4LTIsYI0QfmVUwFYSm1 a9LtCLltPArYlrbbSJ2DkepPyZZBkhHI0ICqdTA/vldIfs+7YztOALZJjLbjp8TR7+zB+MTY6Wn pMu5Inw19JT0w4yb797SNrjwFRPizTthPRgk8kfhVMm3q3vHwv9bDDWTKsJsBc9EyXxMMQ1B41Y zgVn9zf545suxCDN+1QYVzAEFRnqFLAcVmnMmzs+c2paQCcEI1xr9nfpXWllYqUGKloQZtwZJMj NtOOZXO+3F7mk23PQirIGHVRXJGTZpyxKsAfk8vEk3hT8ThFnGqgu+rx/bpatLBLV2wYppiyv3b cKwIiRqezZYy8qvk9aWAWprx2PfhWsFuXB5CMgh/jTkgcJXZF2EV5iOFc7V8vRGejk X-Received: by 2002:a05:600c:1c22:b0:480:20f1:7aa6 with SMTP id 5b1f17b1804b1-4871604c8e1mr76733195e9.21.1774467710858; Wed, 25 Mar 2026 12:41:50 -0700 (PDT) Received: from pumpkin (82-69-66-36.dsl.in-addr.zen.co.uk. [82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4871fbb58a0sm688705e9.23.2026.03.25.12.41.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 Mar 2026 12:41:50 -0700 (PDT) Date: Wed, 25 Mar 2026 19:41:46 +0000 From: David Laight To: Jim Mattson Cc: Pawan Gupta , 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" , 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 , 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: Re: [PATCH v8 02/10] x86/bhi: Make clear_bhb_loop() effective on newer CPUs Message-ID: <20260325194146.29c91953@pumpkin> In-Reply-To: References: <20260324-vmscape-bhb-v8-0-68bb524b3ab9@linux.intel.com> <20260324-vmscape-bhb-v8-2-68bb524b3ab9@linux.intel.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Wed, 25 Mar 2026 10:50:58 -0700 Jim Mattson wrote: > On Tue, Mar 24, 2026 at 11:19=E2=80=AFAM Pawan Gupta > wrote: > > > > As a mitigation for BHI, clear_bhb_loop() executes branches that overwr= ites > > 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 have a hardware control (BHI_DIS_S) > > that mitigates BHI in kernel. > > > > BHI variant of VMSCAPE requires isolating branch history between guests= and > > userspace. Note that there is no equivalent hardware control for usersp= ace. > > To effectively isolate branch history on newer CPUs, clear_bhb_loop() > > should execute sufficient number of branches to clear a larger BHB. > > > > Dynamically set the loop count of clear_bhb_loop() such that it is > > effective on newer CPUs too. Use the hardware control enumeration > > X86_FEATURE_BHI_CTRL to select the appropriate loop count. > > > > Suggested-by: Dave Hansen > > Reviewed-by: Nikolay Borisov > > Signed-off-by: Pawan Gupta > > --- > > arch/x86/entry/entry_64.S | 21 ++++++++++++++++----- > > arch/x86/net/bpf_jit_comp.c | 7 ------- > > 2 files changed, 16 insertions(+), 12 deletions(-) > > > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > > index 3a180a36ca0e..8128e00ca73f 100644 > > --- a/arch/x86/entry/entry_64.S > > +++ b/arch/x86/entry/entry_64.S > > @@ -1535,8 +1535,17 @@ SYM_CODE_END(rewind_stack_and_make_dead) > > SYM_FUNC_START(clear_bhb_loop) > > ANNOTATE_NOENDBR > > push %rbp > > + /* BPF caller may require %rax to be preserved */ Since you need a new version change that to 'all registers preserved'. > > + push %rax =20 >=20 > Shouldn't the "push %rax" come after "mov %rsp, %rbp"? Or delete the stack frame :-) It is only there for the stack trace-back code. David >=20 > > mov %rsp, %rbp > > - movl $5, %ecx =20