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 X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=BAYES_00,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1A313C433E0 for ; Fri, 12 Feb 2021 18:19:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DDC8164DDF for ; Fri, 12 Feb 2021 18:18:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231790AbhBLSS6 (ORCPT ); Fri, 12 Feb 2021 13:18:58 -0500 Received: from mail.kernel.org ([198.145.29.99]:52476 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231720AbhBLSSr (ORCPT ); Fri, 12 Feb 2021 13:18:47 -0500 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id BFB7D64DDF; Fri, 12 Feb 2021 18:18:06 +0000 (UTC) Received: from disco-boy.misterjones.org ([51.254.78.96] helo=www.loen.fr) by disco-boy.misterjones.org with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.94) (envelope-from ) id 1lAd12-00Dsau-MJ; Fri, 12 Feb 2021 18:18:04 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Fri, 12 Feb 2021 18:18:04 +0000 From: Marc Zyngier To: Alexandru Elisei Cc: kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, James Morse , Julien Thierry , Suzuki K Poulose , Will Deacon , kernel-team@android.com, Jianyong Wu Subject: Re: [PATCH] KVM: arm64: Handle CMOs on Read Only memslots In-Reply-To: <4bfd380b-a654-c104-f424-a258bb142e34@arm.com> References: <20210211142738.1478292-1-maz@kernel.org> <4bfd380b-a654-c104-f424-a258bb142e34@arm.com> User-Agent: Roundcube Webmail/1.4.10 Message-ID: <6c127a2d4276b56205d2d28cc0b9ffc2@kernel.org> X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: alexandru.elisei@arm.com, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, will@kernel.org, kernel-team@android.com, jianyong.wu@arm.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Hi Alex, On 2021-02-12 17:12, Alexandru Elisei wrote: > Hi Marc, > > I've been trying to get my head around what the architecture says about > CMOs, so > please bare with me if I misunderstood some things. No worries. I've had this patch for a few weeks now, and can't make up my mind about it. It does address an actual issue though, so I couldn't just discard it... ;-) > On 2/11/21 2:27 PM, Marc Zyngier wrote: >> It appears that when a guest traps into KVM because it is >> performing a CMO on a Read Only memslot, our handling of >> this operation is "slightly suboptimal", as we treat it as >> an MMIO access without a valid syndrome. >> >> The chances that userspace is adequately equiped to deal >> with such an exception being slim, it would be better to >> handle it in the kernel. >> >> What we need to provide is roughly as follows: >> >> (a) if a CMO hits writeable memory, handle it as a normal memory acess >> (b) if a CMO hits non-memory, skip it >> (c) if a CMO hits R/O memory, that's where things become fun: >> (1) if the CMO is DC IVAC, the architecture says this should result >> in a permission fault >> (2) if the CMO is DC CIVAC, it should work similarly to (a) > > When you say it should work similarly to (a), you mean it should be > handled as a > normal memory access, without the "CMO hits writeable memory" part, > right? What I mean is that the cache invalidation should take place, preferably without involving KVM at all (other than populating S2 if required). > >> >> We already perform (a) and (b) correctly, but (c) is a total mess. >> Hence we need to distinguish between IVAC (c.1) and CIVAC (c.2). >> >> One way to do it is to treat CMOs generating a translation fault as >> a *read*, even when they are on a RW memslot. This allows us to >> further triage things: >> >> If they come back with a permission fault, that is because this is >> a DC IVAC instruction: >> - inside a RW memslot: no problem, treat it as a write (a)(c.2) >> - inside a RO memslot: inject a data abort in the guest (c.1) >> >> The only drawback is that DC IVAC on a yet unmapped page faults >> twice: one for the initial translation fault that result in a RO >> mapping, and once for the permission fault. I think we can live with >> that. > > I'm trying to make sure I understand what the problem is. > > gfn_to_pfn_prot() returnsKVM_HVA_ERR_RO_BAD if the write is to a RO > memslot. > KVM_HVA_ERR_RO_BAD is PAGE_OFFSET + PAGE_SIZE, which means that > is_error_noslot_pfn() return true. In that case we exit to userspace > with -EFAULT > for DC IVAC and DC CIVAC. But what we should be doing is this: > > - For DC IVAC, inject a dabt with ISS = 0x10, meaning an external abort > (that's > what kvm_inject_dabt_does()). > > - For DC CIVAC, exit to userspace with -EFAULT. > > Did I get that right? Not quite. What I *think* we should do is: - DC CIVAC should just work, without going to userspace. I can't imagine a reason why we'd involve userspace for this, and we currently don't really have a good way to describe this to userspace. - DC IVAC is more nuanced: we could either inject an exception (which is what this patch does), or perform the CMO ourselves as a DC CIVAC (consistent with the IVA->CIVA upgrade caused by having a S2 translation). This second approach is comparable to what we do when the guest issues a CMO on an emulated MMIO address (we don't inject a fault). Thanks, M. -- Jazz is not dead. It just smells funny...