From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:57697) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fnhAg-0007VJ-7Q for qemu-devel@nongnu.org; Thu, 09 Aug 2018 05:23:55 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fnhAd-0002YR-02 for qemu-devel@nongnu.org; Thu, 09 Aug 2018 05:23:54 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:51078 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fnhAc-0002Y9-RC for qemu-devel@nongnu.org; Thu, 09 Aug 2018 05:23:50 -0400 Date: Thu, 9 Aug 2018 12:23:43 +0300 From: "Michael S. Tsirkin" Message-ID: <20180809122030-mutt-send-email-mst@kernel.org> References: <20180807193125.30378-1-alex.williamson@redhat.com> <20180807224033-mutt-send-email-mst@kernel.org> <20180807135303.3938b3a2@t450s.home> <20180808004929-mutt-send-email-mst@kernel.org> <20180808034543.GC24415@xz-mi> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180808034543.GC24415@xz-mi> Subject: Re: [Qemu-devel] [PATCH v3 0/4] Balloon inhibit enhancements, vfio restriction List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Xu Cc: Alex Williamson , qemu-devel@nongnu.org, Cornelia Huck , kvm@vger.kernel.org, david@redhat.com On Wed, Aug 08, 2018 at 11:45:43AM +0800, Peter Xu wrote: > On Wed, Aug 08, 2018 at 12:58:32AM +0300, Michael S. Tsirkin wrote: > > At least with VTD, it seems entirely possible to change e.g. a PMD > > atomically to point to a different set of PTEs, then flush. > > That will allow removing memory at high granularity for > > an arbitrary device without mdev or PASID dependency. > > My understanding is that the guest driver should prohibit this kind of > operation (say, modifying PMD). Interesting. Which part of the VTD spec prohibits this? > Actually I don't see how it can > happen in Linux if the kernel drivers always call the IOMMU API since > there are only map/unmap APIs rather than this atomic-modify API. It could happen with a non-Linux guest which might have a different API. > The thing is that IMHO it's the guest driver's responsibility to make > sure the pages will never be used by the device before it removes the > entry (including modifying the PMD since that actually removes all the > entries on the old PMD). If you switch PMDs atomically from one set of valid PTEs to another, then flush, then as far as I could see it just works in the hardware VTD, but not in the emulated VTD. So that's a difference in behaviour. Maybe we are lucky and no one does that. > If not, I would see it a guest kernel bug > instead of the bug in the emulation code. > > Thanks, > > -- > Peter Xu