From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To: Nikolay Borisov <nik.borisov@suse.com>
Cc: x86@kernel.org, Jon Kohler <jon@nutanix.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Josh Poimboeuf <jpoimboe@kernel.org>,
David Kaplan <david.kaplan@amd.com>,
Sean Christopherson <seanjc@google.com>,
Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
KP Singh <kpsingh@kernel.org>, Jiri Olsa <jolsa@kernel.org>,
"David S. Miller" <davem@davemloft.net>,
David Laight <david.laight.linux@gmail.com>,
Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
David Ahern <dsahern@kernel.org>,
Martin KaFai Lau <martin.lau@linux.dev>,
Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
Yonghong Song <yonghong.song@linux.dev>,
John Fastabend <john.fastabend@gmail.com>,
Stanislav Fomichev <sdf@fomichev.me>, Hao Luo <haoluo@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Jonathan Corbet <corbet@lwn.net>, Jason Baron <jbaron@akamai.com>,
Alice Ryhl <aliceryhl@google.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ard Biesheuvel <ardb@kernel.org>,
Shuah Khan <skhan@linuxfoundation.org>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Asit Mallick <asit.k.mallick@intel.com>,
Tao Zhang <tao1.zhang@intel.com>,
bpf@vger.kernel.org, netdev@vger.kernel.org,
linux-doc@vger.kernel.org
Subject: Re: [PATCH v12 02/12] x86/bhi: Make clear_bhb_loop() effective on newer CPUs
Date: Wed, 24 Jun 2026 10:49:49 -0700 [thread overview]
Message-ID: <20260624174949.zdcawcbjnlqsimo6@desk> (raw)
In-Reply-To: <171efe97-fd87-45c1-9913-ff62eacab400@suse.com>
On Wed, Jun 24, 2026 at 03:12:28PM +0300, Nikolay Borisov wrote:
>
>
> On 23.06.26 г. 20:33 ч., Pawan Gupta wrote:
> > 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.
> >
> > Introduce global loop counts, initializing them with appropriate value
> > based on the hardware feature X86_FEATURE_BHI_CTRL.
> >
> > Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>
> Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
>
> Although AI brings up a valid argument about whether guests should be
> pessimized and fallback to the longer sequence ?
I don't disagree, but at the same time BHI mitigation for guest migration
is a different beast that should be addressed separately. A series that
adds virtual-SPEC_CTRL support is in the works. Expect the RFC to be posted
in a couple of weeks.
next prev parent reply other threads:[~2026-06-24 17:49 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-06-23 17:32 [PATCH v12 00/12] VMSCAPE optimization for BHI variant Pawan Gupta
2026-06-23 17:32 ` [PATCH v12 01/12] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb_loop() Pawan Gupta
2026-06-23 17:33 ` [PATCH v12 02/12] x86/bhi: Make clear_bhb_loop() effective on newer CPUs Pawan Gupta
2026-06-23 18:13 ` bot+bpf-ci
2026-06-24 12:12 ` Nikolay Borisov
2026-06-24 17:49 ` Pawan Gupta [this message]
2026-06-23 17:33 ` [PATCH v12 03/12] x86/bhi: Rename clear_bhb_loop() to clear_bhb_loop_nofence() Pawan Gupta
2026-06-23 17:33 ` [PATCH v12 04/12] x86/vmscape: Rename x86_ibpb_exit_to_user to x86_predictor_flush_exit_to_user Pawan Gupta
2026-06-23 17:33 ` [PATCH v12 05/12] x86/vmscape: Move mitigation selection to a switch() Pawan Gupta
2026-06-23 17:34 ` [PATCH v12 06/12] x86/vmscape: Use write_ibpb() instead of indirect_branch_prediction_barrier() Pawan Gupta
2026-06-23 17:34 ` [PATCH v12 07/12] static_call: Define EXPORT_STATIC_CALL_FOR_MODULES() Pawan Gupta
2026-06-24 12:59 ` Sean Christopherson
2026-06-23 17:34 ` [PATCH v12 08/12] KVM: Define EXPORT_STATIC_CALL_FOR_KVM() Pawan Gupta
2026-06-23 18:13 ` bot+bpf-ci
2026-06-23 17:34 ` [PATCH v12 09/12] x86/vmscape: Use static_call() for predictor flush Pawan Gupta
2026-06-23 17:35 ` [PATCH v12 10/12] x86/vmscape: Deploy BHB clearing mitigation Pawan Gupta
2026-06-23 17:35 ` [PATCH v12 11/12] x86/vmscape: Resolve conflict between attack-vectors and vmscape=force Pawan Gupta
2026-06-23 18:13 ` bot+bpf-ci
2026-06-23 17:35 ` [PATCH v12 12/12] x86/vmscape: Add cmdline vmscape=on to override attack vector controls Pawan Gupta
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