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Received: from foss.arm.com ([217.140.110.172]) by casper.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1keyqE-0003b5-1I for linux-arm-kernel@lists.infradead.org; Tue, 17 Nov 2020 11:08:27 +0000 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 B9204D6E; Tue, 17 Nov 2020 03:08:02 -0800 (PST) Received: from C02TD0UTHF1T.local (unknown [10.57.27.161]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3AAFE3F718; Tue, 17 Nov 2020 03:08:01 -0800 (PST) Date: Tue, 17 Nov 2020 11:07:58 +0000 From: Mark Rutland To: Catalin Marinas Subject: Re: [PATCHv4 14/17] arm64: uaccess: remove set_fs() Message-ID: <20201117110758.GC73209@C02TD0UTHF1T.local> References: <20201113124937.20574-1-mark.rutland@arm.com> <20201113124937.20574-15-mark.rutland@arm.com> <20201117104454.GA73209@C02TD0UTHF1T.local> <20201117105740.GB73209@C02TD0UTHF1T.local> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201117_110806_574893_BF02F8EB X-CRM114-Status: GOOD ( 30.44 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: james.morse@arm.com, will@kernel.org, hch@lst.de, linux-arm-kernel@lists.infradead.org, robin.murphy@arm.com Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Nov 17, 2020 at 11:02:20AM +0000, Catalin Marinas wrote: > On Tue, Nov 17, 2020 at 10:57:40AM +0000, Mark Rutland wrote: > > On Tue, Nov 17, 2020 at 10:54:18AM +0000, Catalin Marinas wrote: > > > On Tue, Nov 17, 2020 at 10:44:54AM +0000, Mark Rutland wrote: > > > > On Mon, Nov 16, 2020 at 05:40:48PM +0000, Catalin Marinas wrote: > > > > > On Fri, Nov 13, 2020 at 12:49:34PM +0000, Mark Rutland wrote: > > > > > > We no longer need to flip UAO to access kernel memory under KERNEL_DS, > > > > > > and head.S unconditionally clears UAO for all kernel configurations via > > > > > > an ERET in init_kernel_el. Thus, we don't need to dynamically flip UAO, > > > > > > nor do we need to context-switch it. However, we do need to clear UAO > > > > > > (and set PAN) during SDEI entry. > > > > > > > > > > If the kernel never sets the UAO bit, why do we need to clear it during > > > > > SDEI entry? > > > > > > > > The fear was taking an SDEI event from a VM which had UAO set, with the > > > > SDEI FW not clearing UAO. > > > > > > > > That might not happen in practice because while the spec implies that > > > > could happen, TF-A currently generates a new PSTATE from scratch, and > > > > going forward the spec will be aligned with regular exception entry > > > > rules for PSTATE (so UAO will be cleared automatically). > > > > > > Does this requirement apply retrospectively or it only for the future > > > SDEI specs? > > > > It applies from the current spec, but note that TF-A has always > > generated a new PSTATE (with UAO clear). > > And we can assume that TF-A is the only implementation generating SDEI. I *think* so, hopefully James can confirm. > > > > So we can probably drop the clearing of UAO if you prefer. > > > > > > I don't like clearing UAO specifically. There may be other PSTATE bits > > > in the future we don't know or care about and that are left set by > > > firmware. If we don't trust firmware to give a clean PSTATE, can we > > > reset it with an ERET? > > > > In future we should get the new behaviour, so I think we should be fine > > there. If we can't trust PSTATE at all, then we're not necessarily able > > to perform an ERET anyway. > > > > ... which I think means I'm arguing in favour of deleting the clearing > > of UAO, if you're happy with that? > > Yes, I'm fine with that. That means that you can also get rid of > cpu_has_uao(). Yup; I can go adjust the prior patches with that in mind. > Could we apply the same logic to SDEI+PAN? I don't think we can; existing TF-A FW will clear PAN (since it generates the new PSTATE from scratch), so we need to set PAN explicitly to support that. Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel