From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f73.google.com (mail-pj1-f73.google.com [209.85.216.73]) (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 9348518C008 for ; Wed, 29 Oct 2025 22:47:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.73 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761778079; cv=none; b=MLInASbUdgVmbUFs0d7nENieK+Mdu6qsJFnONz1fk4Cy2r7nOovQcU3KJy29QalOXeNZ3JUFRTDjw1kOwDDTFQUqkAsZIwFHZ1USHQuh9cwNhvKRHXImwBG9l5dbKd3tcO08fJe5Q/TTR2BgVy4B9INaKlS+YZApGJPq/mfNEmo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761778079; c=relaxed/simple; bh=cgGZWtGKa5npg/MTX2D6gmhc8SWIDwF/Qcdu1jVtR6o=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=Y3jFLlVfpzbxFtTNu3u++mJN1b8rfdddoPP+sp2cbs2YQnL+gk4EBOdut59AOUC9G2DJ+e3adKUYr9pB7utLHtVeiDuvWSA5G0WucDhakD1HMVO99ruvQriJHH/2n/trqxrg7inU+C765c+RfAudxKhywMTqmjx1lEZi0Wwwp8Y= 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=yVP8piSY; arc=none smtp.client-ip=209.85.216.73 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="yVP8piSY" Received: by mail-pj1-f73.google.com with SMTP id 98e67ed59e1d1-33d75897745so927429a91.0 for ; Wed, 29 Oct 2025 15:47:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1761778076; x=1762382876; 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=6TD8UsDIiiN72tVtcxsjIL/Io+7/pHH4cH58koU9gOo=; b=yVP8piSYCBh82Pm8W04P7Ar+nCCVdFvyG28Q4CLgdvrGERK3WRB4aAOx/LcorcjQqH tx01pBOTTh9/k/G7EDsbaUHUP5YJ+IgpLW/2PnK7iBWAp0Vv1SBXC6Tk80dWqLWHMIZw SWRODfUmPbH4NMxPwzeAPtARm5EOxU636g/cPhzTJjonFIW13IJjCjnaKqZShY305acX DXiBtRtaBtFCAO4+dZlBuGAdE5dyRk+ajAdShtz3elHYmXk0fatYPNzIdDyOYyfFlMFV tEeQ2H0F/q9xvHItKet5FMwf7/hrnrIfpjooSlThcud3NPNEX3fWCPkEX8u29419EU5C S/GQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1761778076; x=1762382876; 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=6TD8UsDIiiN72tVtcxsjIL/Io+7/pHH4cH58koU9gOo=; b=wYBW/2hneOQh4R1+sVSVvqmirYFt03/4g01pbMieIMRXu90fQ073NG5adfNi1PrsNY NDP3+a/POsWc9IboU+F5j9UZLb2A5JckJHY9D89WPef543YfWyFtaqpJyGWa7AIAjvsU DO5e3ZxM4mIb9tBhTl6HakHzzIZiI+WmIBcyYIgLy3eoekj2LoHtkBqJ/6F95Hwgoi44 jbuwneOA0eOS3cq3B6cXmKBeaIbYZ23qQqAD1RnC1pyI3p5LFiFvIMfyqutGizwDu1gC TIuz0R99asN81MteMCxOCh+ZX6OJ0DlsV24rTBy6nKgu0WqZoQ3r2itM03nGf3sginn8 lXhQ== X-Forwarded-Encrypted: i=1; AJvYcCXkEmdj1T2o1lbLUZppr3qILqxocETRKlGorBsP7J7lOlikzw6AucZomqwdSpJ2IVDAnmieidx4BxG+OFI=@vger.kernel.org X-Gm-Message-State: AOJu0YwULr7cBB26XysOEDdaYGKPjQcjOzD6qVO44Ah0gYfAdjO+3jxd h2DDN9CCZpqr72ItDIwZYwQsP0AIn/ynhMurSfWSzlQ1fEykKsPo3TP8LNC0CE8eMQAQaLRDPxl OQM76+g== X-Google-Smtp-Source: AGHT+IFFs/N3Ci15uaXdGWwFKpvhZRCcmQ9q4DWDTfGfBtTXW3KbncWH6bCM5Q4XtD5gsksdTEKAorSW58E= X-Received: from pjte10.prod.google.com ([2002:a17:90a:c20a:b0:32b:35fb:187f]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:4c0e:b0:33b:cb70:31d5 with SMTP id 98e67ed59e1d1-3403a2659ebmr5567686a91.15.1761778075915; Wed, 29 Oct 2025 15:47:55 -0700 (PDT) Date: Wed, 29 Oct 2025 15:47:54 -0700 In-Reply-To: <20251027-vmscape-bhb-v3-2-5793c2534e93@linux.intel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20251027-vmscape-bhb-v3-0-5793c2534e93@linux.intel.com> <20251027-vmscape-bhb-v3-2-5793c2534e93@linux.intel.com> Message-ID: Subject: Re: [PATCH v3 2/3] x86/vmscape: Replace IBPB with branch history clear on exit to userspace From: Sean Christopherson To: Pawan Gupta Cc: x86@kernel.org, "H. Peter Anvin" , Josh Poimboeuf , David Kaplan , Paolo Bonzini , Borislav Petkov , Dave Hansen , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Asit Mallick , Tao Zhang Content-Type: text/plain; charset="us-ascii" On Mon, Oct 27, 2025, Pawan Gupta wrote: > IBPB mitigation for VMSCAPE is an overkill for CPUs that are only affected > by the BHI variant of VMSCAPE. On such CPUs, eIBRS already provides > indirect branch isolation between guest and host userspace. But, a guest > could still poison the branch history. > > To mitigate that, use the recently added clear_bhb_long_loop() to isolate > the branch history between guest and userspace. Add cmdline option > 'vmscape=on' that automatically selects the appropriate mitigation based > on the CPU. > > Acked-by: David Kaplan > Signed-off-by: Pawan Gupta > --- > Documentation/admin-guide/hw-vuln/vmscape.rst | 8 ++++ > Documentation/admin-guide/kernel-parameters.txt | 4 +- > arch/x86/include/asm/cpufeatures.h | 1 + > arch/x86/include/asm/entry-common.h | 12 +++--- > arch/x86/include/asm/nospec-branch.h | 2 +- > arch/x86/kernel/cpu/bugs.c | 53 ++++++++++++++++++------- > arch/x86/kvm/x86.c | 5 ++- > 7 files changed, 61 insertions(+), 24 deletions(-) For the KVM changes, Acked-by: Sean Christopherson