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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49033d44d7dsm62702455e9.5.2026.05.21.03.27.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 21 May 2026 03:27:15 -0700 (PDT) Date: Thu, 21 May 2026 11:27:13 +0100 From: David Laight To: Peter Zijlstra Cc: Marco Elver , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , linux-kernel@vger.kernel.org Subject: Re: [PATCH] x86: Implement _THIS_IP_ using inline asm for 32-bit Message-ID: <20260521112713.0f32beba@pumpkin> In-Reply-To: <20260521070801.GE3126523@noisy.programming.kicks-ass.net> References: <20260521000011.3926424-1-elver@google.com> <20260521070801.GE3126523@noisy.programming.kicks-ass.net> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@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 Thu, 21 May 2026 09:08:01 +0200 Peter Zijlstra wrote: > On Thu, May 21, 2026 at 02:00:09AM +0200, Marco Elver wrote: .. > > #ifdef CONFIG_X86_32 > > #define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0))) > > +#define _THIS_IP_ ({ unsigned long __ip; asm volatile("call 1f\n1: pop %0" : "=r" (__ip)); __ip; }) > > This will mess up the RSB and cause bad performance ripple effects for a > bit each use. Now, I don't think anybody still cares about performance > on 32bit (I certainly don't), so perhaps this is fine. But urgh. Nope, the cpu understands that code sequence and doesn't mess up the RSB. It might even get decoded to a single u-op. Basically it is present at the start of pretty much every PIC function. -- David