From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC PATCH 0/3] Balloon inhibit enhancements Date: Mon, 30 Jul 2018 18:49:58 +0300 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 Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org To: Alex Williamson Return-path: Content-Disposition: inline In-Reply-To: <20180730090137.15917879@t450s.home> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel2=m.gmane.org@nongnu.org Sender: "Qemu-devel" List-Id: 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