From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46647) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aaql4-0000iu-9P for qemu-devel@nongnu.org; Tue, 01 Mar 2016 15:19:03 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aaql1-00050E-3q for qemu-devel@nongnu.org; Tue, 01 Mar 2016 15:19:02 -0500 Received: from thoth.sbs.de ([192.35.17.2]:54465) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aaql0-000502-QU for qemu-devel@nongnu.org; Tue, 01 Mar 2016 15:18:59 -0500 References: <1456078260-6669-1-git-send-email-davidkiarie4@gmail.com> <20160301134419-mutt-send-email-mst@redhat.com> <56D59DA3.3040002@siemens.com> <56D5A069.9030004@siemens.com> <20160301220139-mutt-send-email-mst@redhat.com> From: Jan Kiszka Message-ID: <56D5F8F6.2000001@siemens.com> Date: Tue, 1 Mar 2016 21:17:58 +0100 MIME-Version: 1.0 In-Reply-To: <20160301220139-mutt-send-email-mst@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [V6 0/4] AMD IOMMU List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: valentine.sinitsyn@gmail.com, marcel@redhat.com, David Kiarie , qemu-devel@nongnu.org On 2016-03-01 21:11, Michael S. Tsirkin wrote: > On Tue, Mar 01, 2016 at 03:00:09PM +0100, Jan Kiszka wrote: >> On 2016-03-01 14:48, Jan Kiszka wrote: >>> There is likely no way around write-protecting the IOMMU page tables (in >>> KVM mode) once we evaluated and cached them somewhere. >> >> I mean, when in kvm mode AND having something that caches enabled, of >> course. > > Just write-protecting won't be enough either, since > the moment you remove the protection, all bets are off, > and if you don't, guest will start from the same point > when you re-enter and fault again. We would not remove protection as long as the entry is in use by the IOMMU. There should be no difference from shadow MMU logic here: trap and emulate the write. > > What this seems to call for is a new kind of protection > where yes PTE is write protected, but instead of > making PTE writeable (or killing guest) > KVM handles it as an MMIO: emulates the write and then skips the instruction. > > Emulation can be in kernel, just writing into guest memory > on behalf of the guest - with some kind of notifier > to flush the vfio cache - or instead it can exit to userspace > and have QEMU handle it like MMIO and write into guest memory. Exactly, but that's nothing new, is it? It's "just" slow, like other shadow MMUs. Jan -- Siemens AG, Corporate Technology, CT RDA ITP SES-DE Corporate Competence Center Embedded Linux