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[209.132.183.28]) by mx.google.com with ESMTPS id w9si816419qki.247.2019.03.07.19.41.01 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 07 Mar 2019 19:41:01 -0800 (PST) Received-SPF: pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jglisse@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id E21753C2CF7; Fri, 8 Mar 2019 03:41:00 +0000 (UTC) Received: from redhat.com (ovpn-125-54.rdu2.redhat.com [10.10.125.54]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C72AB1001DC1; Fri, 8 Mar 2019 03:40:55 +0000 (UTC) Date: Thu, 7 Mar 2019 22:40:53 -0500 From: Jerome Glisse To: "Michael S. Tsirkin" Cc: Jason Wang , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, peterx@redhat.com, linux-mm@kvack.org, aarcange@redhat.com Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Message-ID: <20190308034053.GB5562@redhat.com> References: <1551856692-3384-1-git-send-email-jasowang@redhat.com> <1551856692-3384-6-git-send-email-jasowang@redhat.com> <20190307103503-mutt-send-email-mst@kernel.org> <20190307124700-mutt-send-email-mst@kernel.org> <20190307191720.GF3835@redhat.com> <20190307211506-mutt-send-email-mst@kernel.org> <20190308025539.GA5562@redhat.com> <20190307221549-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190307221549-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Fri, 08 Mar 2019 03:41:01 +0000 (UTC) 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 Thu, Mar 07, 2019 at 10:16:00PM -0500, Michael S. Tsirkin wrote: > On Thu, Mar 07, 2019 at 09:55:39PM -0500, Jerome Glisse wrote: > > On Thu, Mar 07, 2019 at 09:21:03PM -0500, Michael S. Tsirkin wrote: > > > On Thu, Mar 07, 2019 at 02:17:20PM -0500, Jerome Glisse wrote: > > > > > It's because of all these issues that I preferred just accessing > > > > > userspace memory and handling faults. Unfortunately there does not > > > > > appear to exist an API that whitelists a specific driver along the lines > > > > > of "I checked this code for speculative info leaks, don't add barriers > > > > > on data path please". > > > > > > > > Maybe it would be better to explore adding such helper then remapping > > > > page into kernel address space ? > > > > > > I explored it a bit (see e.g. thread around: "__get_user slower than > > > get_user") and I can tell you it's not trivial given the issue is around > > > security. So in practice it does not seem fair to keep a significant > > > optimization out of kernel because *maybe* we can do it differently even > > > better :) > > > > Maybe a slightly different approach between this patchset and other > > copy user API would work here. What you want really is something like > > a temporary mlock on a range of memory so that it is safe for the > > kernel to access range of userspace virtual address ie page are > > present and with proper permission hence there can be no page fault > > while you are accessing thing from kernel context. > > > > So you can have like a range structure and mmu notifier. When you > > lock the range you block mmu notifier to allow your code to work on > > the userspace VA safely. Once you are done you unlock and let the > > mmu notifier go on. It is pretty much exactly this patchset except > > that you remove all the kernel vmap code. A nice thing about that > > is that you do not need to worry about calling set page dirty it > > will already be handle by the userspace VA pte. It also use less > > memory than when you have kernel vmap. > > > > This idea might be defeated by security feature where the kernel is > > running in its own address space without the userspace address > > space present. > > Like smap? Yes like smap but also other newer changes, with similar effect, since the spectre drama. Cheers, Jérôme