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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-483a31f0330sm222508405e9.9.2026.02.23.01.42.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 23 Feb 2026 01:42:00 -0800 (PST) Date: Mon, 23 Feb 2026 09:41:58 +0000 From: David Laight To: Thomas Gleixner Cc: Ryan Roberts , Catalin Marinas , Will Deacon , Huacai Chen , Madhavan Srinivasan , Michael Ellerman , Paul Walmsley , Palmer Dabbelt , Albert Ou , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Ingo Molnar , Borislav Petkov , Dave Hansen , Kees Cook , "Gustavo A. R. Silva" , Arnd Bergmann , Mark Rutland , "Jason A. Donenfeld" , Ard Biesheuvel , Jeremy Linton , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH v4 3/3] randomize_kstack: Unify random source across arches Message-ID: <20260223094158.2197d7af@pumpkin> In-Reply-To: <87ecmcwjh9.ffs@tglx> References: <20260119130122.1283821-1-ryan.roberts@arm.com> <20260119130122.1283821-4-ryan.roberts@arm.com> <87ecmcwjh9.ffs@tglx> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-hardening@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 Sun, 22 Feb 2026 22:34:26 +0100 Thomas Gleixner wrote: > On Mon, Jan 19 2026 at 13:01, Ryan Roberts wrote: > > I tested an earlier version of this change on x86 bare metal and it > > showed a smaller but still significant improvement. The bare metal > > system wasn't available this time around so testing was done in a VM > > instance. I'm guessing the cost of rdtsc is higher for VMs. > > No it's not, unless the hypervisor traps RDTSC, which would be insane as > that would cause massive regressions all over the place. > > So guessing is not really helpful if you want to argue performance. The cost of rdtsc will depend on the cpu architecture. To get valid comparisons you need to run on identical systems. Regardless, the cost of rdtsc could easily be larger than the cost of the prandom_u32_state() code (especially if inlined or without all the return thunk 'crap'). David > > Thanks, > > tglx