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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-487208ca5b0sm5779385e9.2.2026.03.25.15.40.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 Mar 2026 15:40:21 -0700 (PDT) Date: Wed, 25 Mar 2026 22:40:18 +0000 From: David Laight To: Borislav Petkov Cc: Pawan Gupta , x86@kernel.org, Jon Kohler , Nikolay Borisov , "H. Peter Anvin" , Josh Poimboeuf , David Kaplan , Sean Christopherson , Dave Hansen , Peter Zijlstra , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , KP Singh , Jiri Olsa , "David S. Miller" , 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 Subject: Re: [PATCH v8 02/10] x86/bhi: Make clear_bhb_loop() effective on newer CPUs Message-ID: <20260325224018.34714c6e@pumpkin> In-Reply-To: <20260325203759.GCacRHp2t8a7c4Bp6E@fat_crate.local> References: <20260324-vmscape-bhb-v8-0-68bb524b3ab9@linux.intel.com> <20260324-vmscape-bhb-v8-2-68bb524b3ab9@linux.intel.com> <20260324205930.GQacL7Mp7vwGBKX1W7@fat_crate.local> <20260324221308.7sh6afdy6r6tsf4w@desk> <20260325203759.GCacRHp2t8a7c4Bp6E@fat_crate.local> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 25 Mar 2026 21:37:59 +0100 Borislav Petkov wrote: > On Tue, Mar 24, 2026 at 03:13:08PM -0700, Pawan Gupta wrote: ... > > Although call to clear_bhb_loop() will be inserted at the end of the BPF > > program before it returns, I am not sure if it is safe to assume that > > trashing registers in the path clear_bhb_loop() -> __clear_bhb_loop() is > > okay? Especially, when we don't know what code compiler generated for > > clear_bhb_loop(). BPF experts would know better? > > The compiler would preserve the regs. If you write it in asm and you adhere to > the C ABI, you could preserve them too. Shouldn't be too many. The BPF code that calls it doesn't use the C ABI - it just puts a call instruction in the code it generates. Hence all registers must be preserved. David > > Thx. > >