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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4356997eb1fsm30869735f8f.35.2026.01.20.10.45.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Jan 2026 10:45:42 -0800 (PST) Date: Tue, 20 Jan 2026 18:45:41 +0000 From: David Laight To: Dave Hansen Cc: Ryan Roberts , Kees Cook , Catalin Marinas , Will Deacon , Huacai Chen , Madhavan Srinivasan , Michael Ellerman , Paul Walmsley , Palmer Dabbelt , Albert Ou , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "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 0/3] Fix bugs and performance of kstack offset randomisation Message-ID: <20260120184541.0a463cd1@pumpkin> In-Reply-To: References: <20260119130122.1283821-1-ryan.roberts@arm.com> <20fb97f1-7e50-485a-bdfd-a2901d20ec84@intel.com> <785a7d90-babe-43af-93a6-c220c1877bcf@arm.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260120_104546_422621_83EA335F X-CRM114-Status: GOOD ( 21.71 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, 20 Jan 2026 08:37:43 -0800 Dave Hansen wrote: > On 1/20/26 08:32, Ryan Roberts wrote: > > I don't think this question was really addressed to me, but I'll give m= y opinion > > anyway; I agree it's pretty binary - it will either work or it will exp= lode. > > I've tested on arm64 and x86_64 so I have high confidence that it works= . If you > > get it into -next ASAP it has 3 weeks to soak before the merge window o= pens > > right? (Linus said he would do an -rc8 this cycle). That feels like eno= ugh time > > to me. But it's your tree =F0=9F=98=89 =20 >=20 > First of all, thank you for testing it on x86! Having that one data > point where it helped performance is super valuable. >=20 > I'm more worried that it's going to regress performance somewhere and > then it's going to be a pain to back out. I'm not super worried about > functional regressions. Unlikely, on x86 the 'rdtsc' is ~20 clocks on Intel cpu and even slower on amd (according to Agner). (That is serialised against another rdtsc rather than other instructions.) Whereas the four TAUSWORTHE() are independent so can execute in parallel. IIRC each is a memory read and 5 ALU instructions - not much at all. The slow bit will be the cache miss on the per-cpu data. You lose a clock at the end because gcc will compile the a | b | c | d as (((a | b) | c) | d) not ((a | b) | (c | d)). I think someone reported the 'new' version being faster on x86, that might be why. 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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4356997eb1fsm30869735f8f.35.2026.01.20.10.45.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Jan 2026 10:45:42 -0800 (PST) Date: Tue, 20 Jan 2026 18:45:41 +0000 From: David Laight To: Dave Hansen Cc: Ryan Roberts , Kees Cook , Catalin Marinas , Will Deacon , Huacai Chen , Madhavan Srinivasan , Michael Ellerman , Paul Walmsley , Palmer Dabbelt , Albert Ou , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "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 0/3] Fix bugs and performance of kstack offset randomisation Message-ID: <20260120184541.0a463cd1@pumpkin> In-Reply-To: References: <20260119130122.1283821-1-ryan.roberts@arm.com> <20fb97f1-7e50-485a-bdfd-a2901d20ec84@intel.com> <785a7d90-babe-43af-93a6-c220c1877bcf@arm.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260120_104546_147726_0B3A253F X-CRM114-Status: GOOD ( 20.10 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org T24gVHVlLCAyMCBKYW4gMjAyNiAwODozNzo0MyAtMDgwMApEYXZlIEhhbnNlbiA8ZGF2ZS5oYW5z ZW5AaW50ZWwuY29tPiB3cm90ZToKCj4gT24gMS8yMC8yNiAwODozMiwgUnlhbiBSb2JlcnRzIHdy b3RlOgo+ID4gSSBkb24ndCB0aGluayB0aGlzIHF1ZXN0aW9uIHdhcyByZWFsbHkgYWRkcmVzc2Vk IHRvIG1lLCBidXQgSSdsbCBnaXZlIG15IG9waW5pb24KPiA+IGFueXdheTsgSSBhZ3JlZSBpdCdz IHByZXR0eSBiaW5hcnkgLSBpdCB3aWxsIGVpdGhlciB3b3JrIG9yIGl0IHdpbGwgZXhwbG9kZS4K PiA+IEkndmUgdGVzdGVkIG9uIGFybTY0IGFuZCB4ODZfNjQgc28gSSBoYXZlIGhpZ2ggY29uZmlk ZW5jZSB0aGF0IGl0IHdvcmtzLiBJZiB5b3UKPiA+IGdldCBpdCBpbnRvIC1uZXh0IEFTQVAgaXQg aGFzIDMgd2Vla3MgdG8gc29hayBiZWZvcmUgdGhlIG1lcmdlIHdpbmRvdyBvcGVucwo+ID4gcmln aHQ/IChMaW51cyBzYWlkIGhlIHdvdWxkIGRvIGFuIC1yYzggdGhpcyBjeWNsZSkuIFRoYXQgZmVl bHMgbGlrZSBlbm91Z2ggdGltZQo+ID4gdG8gbWUuIEJ1dCBpdCdzIHlvdXIgdHJlZSDwn5iJICAK PiAKPiBGaXJzdCBvZiBhbGwsIHRoYW5rIHlvdSBmb3IgdGVzdGluZyBpdCBvbiB4ODYhIEhhdmlu ZyB0aGF0IG9uZSBkYXRhCj4gcG9pbnQgd2hlcmUgaXQgaGVscGVkIHBlcmZvcm1hbmNlIGlzIHN1 cGVyIHZhbHVhYmxlLgo+IAo+IEknbSBtb3JlIHdvcnJpZWQgdGhhdCBpdCdzIGdvaW5nIHRvIHJl Z3Jlc3MgcGVyZm9ybWFuY2Ugc29tZXdoZXJlIGFuZAo+IHRoZW4gaXQncyBnb2luZyB0byBiZSBh IHBhaW4gdG8gYmFjayBvdXQuIEknbSBub3Qgc3VwZXIgd29ycmllZCBhYm91dAo+IGZ1bmN0aW9u YWwgcmVncmVzc2lvbnMuCgpVbmxpa2VseSwgb24geDg2IHRoZSAncmR0c2MnIGlzIH4yMCBjbG9j a3Mgb24gSW50ZWwgY3B1IGFuZCBldmVuIHNsb3dlcgpvbiBhbWQgKGFjY29yZGluZyB0byBBZ25l cikuCihUaGF0IGlzIHNlcmlhbGlzZWQgYWdhaW5zdCBhbm90aGVyIHJkdHNjIHJhdGhlciB0aGFu IG90aGVyIGluc3RydWN0aW9ucy4pCldoZXJlYXMgdGhlIGZvdXIgVEFVU1dPUlRIRSgpIGFyZSBp bmRlcGVuZGVudCBzbyBjYW4gZXhlY3V0ZSBpbiBwYXJhbGxlbC4KSUlSQyBlYWNoIGlzIGEgbWVt b3J5IHJlYWQgYW5kIDUgQUxVIGluc3RydWN0aW9ucyAtIG5vdCBtdWNoIGF0IGFsbC4KVGhlIHNs b3cgYml0IHdpbGwgYmUgdGhlIGNhY2hlIG1pc3Mgb24gdGhlIHBlci1jcHUgZGF0YS4KWW91IGxv c2UgYSBjbG9jayBhdCB0aGUgZW5kIGJlY2F1c2UgZ2NjIHdpbGwgY29tcGlsZSB0aGUgYSB8IGIg fCBjIHwgZAphcyAoKChhIHwgYikgfCBjKSB8IGQpIG5vdCAoKGEgfCBiKSB8IChjIHwgZCkpLgoK SSB0aGluayBzb21lb25lIHJlcG9ydGVkIHRoZSAnbmV3JyB2ZXJzaW9uIGJlaW5nIGZhc3RlciBv biB4ODYsCnRoYXQgbWlnaHQgYmUgd2h5LgoKCURhdmlkCgpfX19fX19fX19fX19fX19fX19fX19f X19fX19fX19fX19fX19fX19fX19fX19fXwpsaW51eC1yaXNjdiBtYWlsaW5nIGxpc3QKbGludXgt cmlzY3ZAbGlzdHMuaW5mcmFkZWFkLm9yZwpodHRwOi8vbGlzdHMuaW5mcmFkZWFkLm9yZy9tYWls bWFuL2xpc3RpbmZvL2xpbnV4LXJpc2N2Cg==