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Rozycki" , "Linus Torvalds" Cc: "Matthew Wilcox" , "Peter Zijlstra" , "the arch/x86 maintainers" , "Yu Zhao" , "Andrew Morton" , "Andi Kleen" , "Aneesh Kumar" , "Catalin Marinas" , "Dave Hansen" , "Hillf Danton" , "Jens Axboe" , "Johannes Weiner" , "Jonathan Corbet" , "Mel Gorman" , "Michael Larabel" , "Michal Hocko" , "Mike Rapoport" , "Tejun Heo" , "Vlastimil Babka" , "Will Deacon" , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, page-reclaim@google.com, "Brian Geffon" , "Jan Alexander Steffens" , "Oleksandr Natalenko" , "Steven Barrett" , "Suleiman Souhlal" , "Daniel Byrne" , "Donald Carr" , =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= , "Konstantin Kharlamov" , "Shuang Zhai" , "Sofia Trinh" , "Vaibhav Jain" Subject: Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks Content-Type: text/plain ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666799069; a=rsa-sha256; cv=none; b=N1Yyx8x/xzoXOJ8zoH8GJ4IVKi5i4dKDFaiTclvQIv/qf4gTdZyr2CLyYAnM7cDBh6gdWt gBiuwVyX7nvH+Hgzj57OB0U4GCBSC2wJo3SVkWFRg5zx5s2pkfBNK0FUI5FL89N61hKYyb q6guoydwcakrvEnpc9K3un+Z9zEILDg= ARC-Authentication-Results: i=1; imf12.hostedemail.com; dkim=pass header.d=arndb.de header.s=fm2 header.b=bJCPweOd; dkim=pass header.d=messagingengine.com header.s=fm3 header.b=Xf94Od4N; dmarc=none; spf=pass (imf12.hostedemail.com: domain of arnd@arndb.de designates 66.111.4.29 as permitted sender) smtp.mailfrom=arnd@arndb.de ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666799069; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=faDhQ6w8KKWL4bKQ9rSJWdVZALluEpOGbDu+CS6W134=; b=wDjUW02Udpqu892UuR6OZ0FRcUKITFzvi5HhAtCQBjg6FY+uE8l2/77i4k78/I0VnFsesT Avarz6fVDGCHnP+yQV50kXCoCTrQUqpWbP/te8xgKTnWh5M/P3PoqkSi2kCmvj5zvrajtS ZjoCEpmIf+WiT25WlZ9txy7KB+Qd00I= X-Stat-Signature: a5tyfbzebgpnahjbffefsxgxzo79ow3u X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: D9A4040034 Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=arndb.de header.s=fm2 header.b=bJCPweOd; dkim=pass header.d=messagingengine.com header.s=fm3 header.b=Xf94Od4N; dmarc=none; spf=pass (imf12.hostedemail.com: domain of arnd@arndb.de designates 66.111.4.29 as permitted sender) smtp.mailfrom=arnd@arndb.de X-HE-Tag: 1666799068-960012 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Oct 25, 2022, at 18:28, Maciej W. Rozycki wrote: > On Sun, 23 Oct 2022, Linus Torvalds wrote: > >> In fact, I don't understand how current kernels work on an i486 at >> all, since it looks like >> >> exit_to_user_mode_prepare -> >> arch_exit_to_user_mode_prepare >> >> ends up having an unconditional 'rdtsc' instruction in it. >> > > The fix here is obviously and trivially: > > select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET if !M486SX && !M486 I think that would be "if X86_TSC", otherwise you still include the TSC-less 586-class (5x86, 6x86, Elan, Winchip C6, MediaGX, ...) > So what's the actual burden from keeping this support around? Would my > proposal to emulate CMPXCHG8B (and possibly RDTSC) in #UD handler help? That sounds worse to me than the current use of runtime alternatives for picking between cmpxchg8b_emu and the native instruction. For arm32, we have a combination of two other approaches: - On the oldest processors that never had SMP support (ARMv5 and earlier), it is not possible to enable support for SMP at all. Using a Kconfig 'depends on X86_CMPXCHG64' for CONFIG_SMP would still allow building 486 kernels, but completely avoid the problem of trying to make the same kernel work on later SMP machines. - For the special case of early ARMv6 hardware that has 32-bit atomics but not 64-bit ones, the kernel just falls back to CONFIG_GENERIC_ATOMIC64 and no cmpxchg64(). The same should work for an i486+SMP kernel. It's obviously slower, but most users can trivially avoid this by either running an i686 SMP kernel or an i486 UP kernel. Arnd