From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To: x86@kernel.org, Jon Kohler <jon@nutanix.com>,
Nikolay Borisov <nik.borisov@suse.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>
Cc: 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 v10 01/12] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb_loop()
Date: Tue, 14 Apr 2026 11:05:00 -0700 [thread overview]
Message-ID: <20260414180442.gcio7h6zjwjcayrs@desk> (raw)
In-Reply-To: <20260414-vmscape-bhb-v10-1-efa924abae5f@linux.intel.com>
On Tue, Apr 14, 2026 at 12:05:28AM -0700, Pawan Gupta wrote:
> Currently, the BHB clearing sequence is followed by an LFENCE to prevent
> transient execution of subsequent indirect branches prematurely. However,
> the LFENCE barrier could be unnecessary in certain cases. For example, when
> the kernel is using the BHI_DIS_S mitigation, and BHB clearing is only
> needed for userspace. In such cases, the LFENCE is redundant because ring
> transitions would provide the necessary serialization.
>
> Below is a quick recap of BHI mitigation options:
>
> On Alder Lake and newer
>
> BHI_DIS_S: Hardware control to mitigate BHI in ring0. This has low
> performance overhead.
>
> Long loop: Alternatively, a longer version of the BHB clearing sequence
> can be used to mitigate BHI. It can also be used to mitigate the BHI
> variant of VMSCAPE. This is not yet implemented in Linux.
>
> On older CPUs
>
> Short loop: Clears BHB at kernel entry and VMexit. The "Long loop" is
> effective on older CPUs as well, but should be avoided because of
> unnecessary overhead.
>
> On Alder Lake and newer CPUs, eIBRS isolates the indirect targets between
> guest and host. But when affected by the BHI variant of VMSCAPE, a guest's
> branch history may still influence indirect branches in userspace. This
> also means the big hammer IBPB could be replaced with a cheaper option that
> clears the BHB at exit-to-userspace after a VMexit.
>
> In preparation for adding the support for the BHB sequence (without LFENCE)
> on newer CPUs, move the LFENCE to the caller side after clear_bhb_loop() is
> executed. Allow callers to decide whether they need the LFENCE or not. This
> adds a few extra bytes to the call sites, but it obviates the need for
> multiple variants of clear_bhb_loop().
>
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Tested-by: Jon Kohler <jon@nutanix.com>
> Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> ---
Sorry this is missing Boris's Ack, I will fix.
> Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
next prev parent reply other threads:[~2026-04-14 18:05 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-14 7:05 [PATCH v10 00/12] VMSCAPE optimization for BHI variant Pawan Gupta
2026-04-14 7:05 ` [PATCH v10 01/12] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb_loop() Pawan Gupta
2026-04-14 18:05 ` Pawan Gupta [this message]
2026-04-14 7:05 ` [PATCH v10 02/12] x86/bhi: Make clear_bhb_loop() effective on newer CPUs Pawan Gupta
2026-04-14 7:06 ` [PATCH v10 03/12] x86/bhi: Rename clear_bhb_loop() to clear_bhb_loop_nofence() Pawan Gupta
2026-04-14 7:06 ` [PATCH v10 04/12] x86/vmscape: Rename x86_ibpb_exit_to_user to x86_predictor_flush_exit_to_user Pawan Gupta
2026-04-14 7:06 ` [PATCH v10 05/12] x86/vmscape: Move mitigation selection to a switch() Pawan Gupta
2026-04-14 7:06 ` [PATCH v10 06/12] x86/vmscape: Use write_ibpb() instead of indirect_branch_prediction_barrier() Pawan Gupta
2026-04-14 7:07 ` [PATCH v10 07/12] static_call: Add EXPORT_STATIC_CALL_FOR_MODULES() Pawan Gupta
2026-04-14 7:07 ` [PATCH v10 08/12] kvm: Define EXPORT_STATIC_CALL_FOR_KVM() Pawan Gupta
2026-04-16 22:44 ` Sean Christopherson
2026-04-16 23:12 ` Pawan Gupta
2026-04-14 7:07 ` [PATCH v10 09/12] x86/vmscape: Use static_call() for predictor flush Pawan Gupta
2026-04-16 22:45 ` Sean Christopherson
2026-04-14 7:07 ` [PATCH v10 10/12] x86/vmscape: Deploy BHB clearing mitigation Pawan Gupta
2026-04-14 7:08 ` [PATCH v10 11/12] x86/vmscape: Resolve conflict between attack-vectors and vmscape=force Pawan Gupta
2026-04-14 7:08 ` [PATCH v10 12/12] x86/vmscape: Add cmdline vmscape=on to override attack vector controls Pawan Gupta
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