From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55800) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fkAQw-00085v-Mk for qemu-devel@nongnu.org; Mon, 30 Jul 2018 11:50:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fkAQs-0002GQ-Ie for qemu-devel@nongnu.org; Mon, 30 Jul 2018 11:50:06 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:52138 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 1fkAQs-0002GF-FC for qemu-devel@nongnu.org; Mon, 30 Jul 2018 11:50:02 -0400 Date: Mon, 30 Jul 2018 18:49:58 +0300 From: "Michael S. Tsirkin" Message-ID: <20180730182951-mutt-send-email-mst@kernel.org> References: <20180717222721.14019.27548.stgit@gimli.home> <20180730163123-mutt-send-email-mst@kernel.org> <20180730083939.23f3b9cc@t450s.home> <20180730174601-mutt-send-email-mst@kernel.org> <20180730090137.15917879@t450s.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180730090137.15917879@t450s.home> Subject: Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alex Williamson Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org On Mon, Jul 30, 2018 at 09:01:37AM -0600, Alex Williamson wrote: > > > but I don't think it can be done > > > atomically with respect to inflight DMA of a physical device where we > > > cannot halt the device without interfering with its state. > > > > Guests never add pages to the balloon if they are under DMA, > > so that's fine - there's never an in-flight DMA, if > > there is guest is buggy and it's ok to crash it. > > It's not the ballooned page that I'm trying to note, it's the entire > remainder of the SubRegion which needs to be unmapped to remove that > one page. It's more compatible from an IOMMU perspective in that we're > only unmapping with the same granularity with which we mapped, but it's > incompatible with inflight DMA as we have no idea what DMA targets may > reside within the remainder of that mapping while it's temporarily > unmapped. I see. Yes you need to be careful to replace the host IOMMU PTE atomically. Same applies to vIOMMU though - if guest changes a PTE atomically host should do the same. -- MST