From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ed1-f52.google.com (mail-ed1-f52.google.com [209.85.208.52]) (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 963131FF7C8 for ; Wed, 10 Dec 2025 12:31:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.208.52 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765369897; cv=none; b=WUQdZcvZvm6DxihiMlCvSQXY3PWokClP5UpUqN5FGv4GEuDgPKR1OGaYCPsgzN+OzLhi2Qte0YQIaS3gTwxGdmQgtgraGpmXDhhLth1IdMU0Db+g8gQAj6SO2ZmdNERC5zwRKlTrXWMzQJ4BiIlPi36E3KXzBvkfeJiP/nwYJ+E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765369897; c=relaxed/simple; bh=EnxQ7+j7c0/T9vlpjPn2CKwVrPO89szqVESVHadPlL4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=eQ7N8oWUsrlnpclNXft0LEMwTJuTvRUh9rabW3MmrDZevNZ8rGd81e6puTqH82i3uJVqMLNdxs7n1eWb0kooRqB+aD6YelxZ+geeo164jYH2ITFZiH0xzYzwWK3nByWqUe2U3A+cZSprK7qLCGglH9CgYAuH9A+UTX2wzK1q1LM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=suse.com; spf=pass smtp.mailfrom=suse.com; dkim=pass (2048-bit key) header.d=suse.com header.i=@suse.com header.b=A1rVkAK6; arc=none smtp.client-ip=209.85.208.52 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=suse.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=suse.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=suse.com header.i=@suse.com header.b="A1rVkAK6" Received: by mail-ed1-f52.google.com with SMTP id 4fb4d7f45d1cf-6495c4577adso2563504a12.3 for ; Wed, 10 Dec 2025 04:31:35 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=google; t=1765369894; x=1765974694; darn=vger.kernel.org; h=content-transfer-encoding:in-reply-to:autocrypt:content-language :from:references:cc:to:subject:user-agent:mime-version:date :message-id:from:to:cc:subject:date:message-id:reply-to; bh=eVr/YoB9YqZwfyTNCf6R5H8DydHWbJpQ1eeeaAEh2UE=; b=A1rVkAK67MNjjAWT3NkGZJESxAa6kYG8r0sXjyRbvtlytcP24ujNe4ZdVgm7IjH4Eq BCm/YD7FLWssR3ybKMXRtJTssyP8Wj5XVVqdIkvz4eo/y0AYAFZX5PEvJ6V/SgAA/Azd qRbayEEBxBw1RDiIPieJYvjkdAA54FO/p+g61B97yxZIMbxK1AvsbHoEEktwZYhsCxPn pN6kaKnGwvYTP1AoMbttGzRSGVe9d0SW3ljyLFRjvLZikjYrGnCCgnPx8nooGNZEnRij Fzqg3BWSyS8IUhOVseB8wYkuEILDtqJ1ZCUOZLg/JeZ+eFHg3WyBeBLHXR1UCoz99+MC zgog== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1765369894; x=1765974694; h=content-transfer-encoding:in-reply-to:autocrypt:content-language :from:references:cc:to:subject:user-agent:mime-version:date :message-id:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=eVr/YoB9YqZwfyTNCf6R5H8DydHWbJpQ1eeeaAEh2UE=; b=nCDttM83g23JpxwukdKlbW5vwLsR/VOLi/FEpUE4MBnZPpwr3GIA2qj+KJPCIAG52E aV1eBZH3I7bGKhs2VyQtuQOFhXH/0SgvB2k/4SN5GlR2AWpRK/hQzH0MPtIT9bjLnOM+ nOGzrKPiMOUW0KQasy/EcJU56hjAEAlMGEF3A0j7nZWNx91AggWIBMZTWm9YOW1xjB9e nBDFYdlprVD2Ow5odCHxKx+WJKeXOvR8crCpXeolK4OAs0GMVW6v139L8dx+gNVBqSJ8 SbTA7RYdUIlHTU0KjF2Szcay7GTYSAoYsT5Lb3+Ph56ckjnTPfV9phGu9noMrHz1FyZG XVoA== X-Gm-Message-State: AOJu0YzJZGB4jUBxaCvVvzu3nX+zpYjrzN4RdxMlKvdHqbOgi0IweNS5 7mtPK7EijrUh9Jws4pPWDv3uCPXdR+I8U0+5036CkwwdHk2e65nbjyAhWOXfr9/sZZgWgzlUSLy lchBr X-Gm-Gg: AY/fxX4ndyIwz8q3KYpXhAmre5KKrYlb7j4lbVl8Cpr76vfhnAtJft98QuiK3Ug38WN hN4T4Q66BdXKNw0Kib3k3aCzPbA5HzbdzctZ1O8ZnaA1UaKZKeOvmzph6BEHpoVsqze2jM+Zn41 QGHH0nlTNvAXXSOfHiayVtbEl7CaxRtV7foS50flHyL0qS1atXcE/B3R9L6xJjQxCCGZ4bfRVbq cSQ5elI3YxA8HzT8qXZfHE8Ff1YmQ+/5dDq/0vQdAeKPVDcT4zNXRw1rG/7E0z+5k2j9d3Chr/l LAy+GhI5ZFWs22zf98Px/VVer2cb8FaYk6+dmO7ZTJlepYtnT6mm//dbsN2Eu4KTYOT42NeaDrt iGah0MzHAlgpk1WP8Lx6GrGYSiPc1I0IVJXyAngbRJJGGtaMWQeDSD5zUImNhsuQZJ2oO0v3y2i gswDlftdN2SjGZRg== X-Google-Smtp-Source: AGHT+IHLw3qSVktTGwEHIgnTe7O2rSBAPhtt5q8MfURnABHy8XYIpIgwk83c2V039BPtulBr4rfgHw== X-Received: by 2002:a05:6402:1941:b0:640:c8b7:f2bd with SMTP id 4fb4d7f45d1cf-6496cbe463emr1811758a12.29.1765369893790; Wed, 10 Dec 2025 04:31:33 -0800 (PST) Received: from [192.168.0.20] ([212.21.133.30]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-647b412146bsm16725327a12.23.2025.12.10.04.31.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Dec 2025 04:31:33 -0800 (PST) Message-ID: Date: Wed, 10 Dec 2025 14:31:31 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 2/9] x86/bhi: Make clear_bhb_loop() effective on newer CPUs To: Pawan Gupta , x86@kernel.org, David Kaplan , Nikolay Borisov , "H. Peter Anvin" , Josh Poimboeuf , Sean Christopherson , Paolo Bonzini , Borislav Petkov , Dave Hansen Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Asit Mallick , Tao Zhang References: <20251201-vmscape-bhb-v6-0-d610dd515714@linux.intel.com> <20251201-vmscape-bhb-v6-2-d610dd515714@linux.intel.com> From: Nikolay Borisov Content-Language: en-US Autocrypt: addr=nik.borisov@suse.com; keydata= xsFNBGcrpvIBEAD5cAR5+qu30GnmPrK9veWX5RVzzbgtkk9C/EESHy9Yz0+HWgCVRoNyRQsZ 7DW7vE1KhioDLXjDmeu8/0A8u5nFMqv6d1Gt1lb7XzSAYw7uSWXLPEjFBtz9+fBJJLgbYU7G OpTKy6gRr6GaItZze+r04PGWjeyVUuHZuncTO7B2huxcwIk9tFtRX21gVSOOC96HcxSVVA7X N/LLM2EOL7kg4/yDWEhAdLQDChswhmdpHkp5g6ytj9TM8bNlq9I41hl/3cBEeAkxtb/eS5YR 88LBb/2FkcGnhxkGJPNB+4Siku7K8Mk2Y6elnkOctJcDvk29DajYbQnnW4nhfelZuLNupb1O M0912EvzOVI0dIVgR+xtosp66bYTOpX4Xb0fylED9kYGiuEAeoQZaDQ2eICDcHPiaLzh+6cc pkVTB0sXkWHUsPamtPum6/PgWLE9vGI5s+FaqBaqBYDKyvtJfLK4BdZng0Uc3ijycPs3bpbQ bOnK9LD8TYmYaeTenoNILQ7Ut54CCEXkP446skUMKrEo/HabvkykyWqWiIE/UlAYAx9+Ckho TT1d2QsmsAiYYWwjU8igXBecIbC0uRtF/cTfelNGrQwbICUT6kJjcOTpQDaVyIgRSlUMrlNZ XPVEQ6Zq3/aENA8ObhFxE5PLJPizJH6SC89BMKF3zg6SKx0qzQARAQABzSZOaWtvbGF5IEJv cmlzb3YgPG5pay5ib3Jpc292QHN1c2UuY29tPsLBkQQTAQoAOxYhBDuWB8EJLBUZCPjT3SRn XZEnyhfsBQJnK6byAhsDBQsJCAcCAiICBhUKCQgLAgQWAgMBAh4HAheAAAoJECRnXZEnyhfs XbIQAJxuUnelGdXbSbtovBNm+HF3LtT0XnZ0+DoR0DemUGuA1bZAlaOXGr5mvVbTgaoGUQIJ 3Ejx3UBEG7ZSJcfJobB34w1qHEDO0pN9orGIFT9Bic3lqhawD2r85QMcWwjsZH5FhyRx7P2o DTuUClLMO95GuHYQngBF2rHHl8QMJPVKsR18w4IWAhALpEApxa3luyV7pAAqKllfCNt7tmed uKmclf/Sz6qoP75CvEtRbfAOqYgG1Uk9A62C51iAPe35neMre3WGLsdgyMj4/15jPYi+tOUX Tc7AAWgc95LXyPJo8069MOU73htZmgH4OYy+S7f+ArXD7h8lTLT1niff2bCPi6eiAQq6b5CJ Ka4/27IiZo8tm1XjLYmoBmaCovqx5y5Xt2koibIWG3ZGD2I+qRwZ0UohKRH6kKVHGcrmCv0J YO8yIprxgoYmA7gq21BpTqw3D4+8xujn/6LgndLKmGESM1FuY3ymXgj5983eqaxicKpT9iq8 /a1j31tms4azR7+6Dt8H4SagfN6VbJ0luPzobrrNFxUgpjR4ZyQQ++G7oSRdwjfIh1wuCF6/ mDUNcb6/kA0JS9otiC3omfht47yQnvod+MxFk1lTNUu3hePJUwg1vT1te3vO5oln8lkUo9BU knlYpQ7QA2rDEKs+YWqUstr4pDtHzwQ6mo0rqP+zzsFNBGcrpvIBEADGYTFkNVttZkt6e7yA LNkv3Q39zQCt8qe7qkPdlj3CqygVXfw+h7GlcT9fuc4kd7YxFys4/Wd9icj9ZatGMwffONmi LnUotIq2N7+xvc4Xu76wv+QJpiuGEfCDB+VdZOmOzUPlmMkcJc/EDSH4qGogIYRu72uweKEq VfBI43PZIGpGJ7TjS3THX5WVI2YNSmuwqxnQF/iVqDtD2N72ObkBwIf9GnrOgxEyJ/SQq2R0 g7hd6IYk7SOKt1a8ZGCN6hXXKzmM6gHRC8fyWeTqJcK4BKSdX8PzEuYmAJjSfx4w6DoxdK5/ 9sVrNzaVgDHS0ThH/5kNkZ65KNR7K2nk45LT5Crjbg7w5/kKDY6/XiXDx7v/BOR/a+Ryo+lM MffN3XSnAex8cmIhNINl5Z8CAvDLUtItLcbDOv7hdXt6DSyb65CdyY8JwOt6CWno1tdjyDEG 5ANwVPYY878IFkOJLRTJuUd5ltybaSWjKIwjYJfIXuoyzE7OL63856MC/Os8PcLfY7vYY2LB cvKH1qOcs+an86DWX17+dkcKD/YLrpzwvRMur5+kTgVfXcC0TAl39N4YtaCKM/3ugAaVS1Mw MrbyGnGqVMqlCpjnpYREzapSk8XxbO2kYRsZQd8J9ei98OSqgPf8xM7NCULd/xaZLJUydql1 JdSREId2C15jut21aQARAQABwsF2BBgBCgAgFiEEO5YHwQksFRkI+NPdJGddkSfKF+wFAmcr pvICGwwACgkQJGddkSfKF+xuuxAA4F9iQc61wvAOAidktv4Rztn4QKy8TAyGN3M8zYf/A5Zx VcGgX4J4MhRUoPQNrzmVlrrtE2KILHxQZx5eQyPgixPXri42oG5ePEXZoLU5GFRYSPjjTYmP ypyTPN7uoWLfw4TxJqWCGRLsjnkwvyN3R4161Dty4Uhzqp1IkNhl3ifTDYEvbnmHaNvlvvna 7+9jjEBDEFYDMuO/CA8UtoVQXjy5gtOhZZkEsptfwQYc+E9U99yxGofDul7xH41VdXGpIhUj 4wjd3IbgaCiHxxj/M9eM99ybu5asvHyMo3EFPkyWxZsBlUN/riFXGspG4sT0cwOUhG2ZnExv XXhOGKs/y3VGhjZeCDWZ+0ZQHPCL3HUebLxW49wwLxvXU6sLNfYnTJxdqn58Aq4sBXW5Un0Q vfbd9VFV/bKFfvUscYk2UKPi9vgn1hY38IfmsnoS8b0uwDq75IBvup9pYFyNyPf5SutxhFfP JDjakbdjBoYDWVoaPbp5KAQ2VQRiR54lir/inyqGX+dwzPX/F4OHfB5RTiAFLJliCxniKFsM d8eHe88jWjm6/ilx4IlLl9/MdVUGjLpBi18X7ejLz3U2quYD8DBAGzCjy49wJ4Di4qQjblb2 pTXoEyM2L6E604NbDu0VDvHg7EXh1WwmijEu28c/hEB6DwtzslLpBSsJV0s1/jE= In-Reply-To: <20251201-vmscape-bhb-v6-2-d610dd515714@linux.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2.12.25 г. 8:19 ч., Pawan Gupta wrote: > As a mitigation for BHI, clear_bhb_loop() executes branches that overwrites > 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 userspace. > 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 nit: My RB tag is incorrect, while I did agree with Dave's suggestion to have global variables for the loop counts I haven't' really seen the code so I couldn't have given my RB on something which I haven't seen but did agree with in principle. Now that I have seen the code I'm willing to give my : Reviewed-by: Nikolay Borisov > --- > arch/x86/entry/entry_64.S | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > index 886f86790b4467347031bc27d3d761d5cc286da1..9f6f4a7c5baf1fe4e3ab18b11e25e2fbcc77489d 100644 > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -1536,7 +1536,11 @@ SYM_FUNC_START(clear_bhb_loop) > ANNOTATE_NOENDBR > push %rbp > mov %rsp, %rbp > - movl $5, %ecx > + > + /* loop count differs based on BHI_CTRL, see Intel's BHI guidance */ > + ALTERNATIVE "movl $5, %ecx; movl $5, %edx", \ > + "movl $12, %ecx; movl $7, %edx", X86_FEATURE_BHI_CTRL nit: Just > + > ANNOTATE_INTRA_FUNCTION_CALL > call 1f > jmp 5f > @@ -1557,7 +1561,7 @@ SYM_FUNC_START(clear_bhb_loop) > * but some Clang versions (e.g. 18) don't like this. > */ > .skip 32 - 18, 0xcc > -2: movl $5, %eax > +2: movl %edx, %eax > 3: jmp 4f > nop > 4: sub $1, %eax >