From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0A6AAC531DC for ; Tue, 20 Aug 2024 14:45:59 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [127.0.0.1]) by lists.ozlabs.org (Postfix) with ESMTP id 4WpC2G3xrcz2yDS; Wed, 21 Aug 2024 00:45:58 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: lists.ozlabs.org; arc=none smtp.remote-ip=217.140.110.172 Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=arm.com (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=dave.martin@arm.com; receiver=lists.ozlabs.org) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lists.ozlabs.org (Postfix) with ESMTP id 4WpC2G0zTcz2y92 for ; Wed, 21 Aug 2024 00:45:57 +1000 (AEST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id B7AD6DA7; Tue, 20 Aug 2024 07:45:52 -0700 (PDT) Received: from e133380.arm.com (e133380.arm.com [10.1.197.59]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B2E113F66E; Tue, 20 Aug 2024 07:45:23 -0700 (PDT) Date: Tue, 20 Aug 2024 15:45:21 +0100 From: Dave Martin To: Joey Gouly Cc: Catalin Marinas , linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, aneesh.kumar@kernel.org, aneesh.kumar@linux.ibm.com, bp@alien8.de, broonie@kernel.org, christophe.leroy@csgroup.eu, dave.hansen@linux.intel.com, hpa@zytor.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, maz@kernel.org, mingo@redhat.com, mpe@ellerman.id.au, naveen.n.rao@linux.ibm.com, npiggin@gmail.com, oliver.upton@linux.dev, shuah@kernel.org, szabolcs.nagy@arm.com, tglx@linutronix.de, will@kernel.org, x86@kernel.org, kvmarm@lists.linux.dev Subject: Re: [PATCH v4 18/29] arm64: add POE signal support Message-ID: References: <20240806103532.GA1986436@e124191.cambridge.arm.com> <20240806143103.GB2017741@e124191.cambridge.arm.com> <20240815131815.GA3657684@e124191.cambridge.arm.com> <20240820095441.GA688664@e124191.cambridge.arm.com> <20240820140606.GA1011855@e124191.cambridge.arm.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Post: List-Subscribe: , , List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240820140606.GA1011855@e124191.cambridge.arm.com> On Tue, Aug 20, 2024 at 03:06:06PM +0100, Joey Gouly wrote: > On Tue, Aug 20, 2024 at 02:54:50PM +0100, Dave Martin wrote: > > On Tue, Aug 20, 2024 at 10:54:41AM +0100, Joey Gouly wrote: > > > On Mon, Aug 19, 2024 at 06:09:06PM +0100, Catalin Marinas wrote: > > > > On Thu, Aug 15, 2024 at 04:09:26PM +0100, Dave P Martin wrote: > > > > > On Thu, Aug 15, 2024 at 02:18:15PM +0100, Joey Gouly wrote: > > > > > > That's a lot of words to say, or ask, do you agree with the approach of only > > > > > > saving POR_EL0 in the signal frame if num_allocated_pkeys() > 1? > > > > > > > > > > > > Thanks, > > > > > > Joey > > > > > > > > > > ...So..., given all the above, it is perhaps best to go back to > > > > > dumping POR_EL0 unconditionally after all, unless we have a mechanism > > > > > to determine whether pkeys are in use at all. > > > > > > > > Ah, I can see why checking for POR_EL0_INIT is useful. Only checking for > > > > the allocated keys gets confusing with pkey 0. > > > > > > > > Not sure what the deal is with pkey 0. Is it considered allocated by > > > > default or unallocatable? If the former, it implies that pkeys are > > > > already in use (hence the additional check for POR_EL0_INIT). In > > > > principle the hardware allows us to use permissions where the pkeys do > > > > not apply but we'd run out of indices and PTE bits to encode them, so I > > > > think by default we should assume that pkey 0 is pre-allocated. > > > > > > > > > > > > > > You can consider pkey 0 allocated by default. You can actually pkey_free(0), there's nothing stopping that. > > > > Is that intentional? > > I don't really know? It's intentional from my side in that it, I allow it, > because it doesn't look like x86 or PPC block pkey_free(0). > > I found this code that does pkey_free(0), but obviously it's a bit of a weird test case: > > https://github.com/ColinIanKing/stress-ng/blob/master/test/test-pkey-free.c#L29 Of course, pkey 0 will still be in use for everything, and if the man pages are to be believed, the PKRU bits for pkey 0 may no longer be maintained after this call... So this test is possibly a little braindead. A clear use-case for freeing pkey 0 would be more convincing. In the meantime though, it makes most sense for arm64 to follow the precedent set by other arches on this (as you did). [...] Cheers ---Dave