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AJvYcCXaIBSNjN0EZUnYr+JoEnpWXI3GNDw1j0zILXUfnxtejEbqn60XULJHhrjGdvl0GsGLDROpdpo=@vger.kernel.org X-Gm-Message-State: AOJu0Yxc+eTal05K7u2+IPcVlLqzDUFhND51j6gG9oyRMSO7ggPOAA1K +EmMCroPICZlrktrPv0IWy1aPX7o7TybB25k6vbX/BiCuUqoxV5gsDwX4tn50RPQvQHLPcyHy6Q 84HNdmL+eKiUioZlFObuXvW+AHYXS0NsKL8JGidH0 X-Gm-Gg: ATEYQzwtSGRwdypJEIEPT1WJGQeh1q4utas6K+xfOFjk0akcvwyLuT6DH4Y5VTnAtD9 Lr5R/87Czbe2413mSzo4GX5hTn4/txLZRxmXhI59X+P+86Y6GCJlE/YVugknrDt2L2jdcV2Wh48 3jZXpABgG0F70FBds4e250RKhaB5S67zhxH6hPQ4o7dlrwp959cS8u8pfey5fW3pVQsgRWPb2+K nzOby6oY+8zynMkzpyoBQVWH2A0uAVl4hqcJsKcFXdSb+g1OjFHXWhSQGkBKO436XMuVJL6ZEQB d9gcJT4= X-Received: by 2002:a05:6402:1451:b0:668:c2b6:9fa2 with SMTP id 4fb4d7f45d1cf-66ad9575059mr2781a12.7.1774461072029; Wed, 25 Mar 2026 10:51:12 -0700 (PDT) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20260324-vmscape-bhb-v8-0-68bb524b3ab9@linux.intel.com> <20260324-vmscape-bhb-v8-2-68bb524b3ab9@linux.intel.com> In-Reply-To: <20260324-vmscape-bhb-v8-2-68bb524b3ab9@linux.intel.com> From: Jim Mattson Date: Wed, 25 Mar 2026 10:50:58 -0700 X-Gm-Features: AQROBzDwHlJH5nkrozA8k_jCaaxMY2azgH1uzVE_N9olWzdVjI8yDoxoWPHYtxY Message-ID: Subject: Re: [PATCH v8 02/10] x86/bhi: Make clear_bhb_loop() effective on newer CPUs To: Pawan Gupta Cc: 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" , David Laight , 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable 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 overwrit= es > 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 a= nd > userspace. Note that there is no equivalent hardware control for userspac= e. > 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 */ > + push %rax Shouldn't the "push %rax" come after "mov %rsp, %rbp"? > mov %rsp, %rbp > - movl $5, %ecx