From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH v4 0/6] virtio core DMA API conversion Date: Mon, 9 Nov 2015 13:27:57 +0100 Message-ID: <5640914D.6070401__48321.2188155905$1447072106$gmane$org@redhat.com> References: <20151109133624-mutt-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20151109133624-mutt-send-email-mst@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: "Michael S. Tsirkin" , Andy Lutomirski Cc: linux-s390 , Joerg Roedel , KVM , benh@kernel.crashing.org, Sebastian Ott , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, Christian Borntraeger , sparclinux@vger.kernel.org, dwmw2@infradead.org, Christoph Hellwig , "David S. Miller" , Martin Schwidefsky List-Id: virtualization@lists.linuxfoundation.org On 09/11/2015 13:15, Michael S. Tsirkin wrote: > Well that's not exactly true. I think we would like to make > it possible to put virtio devices behind an IOMMU on x86, > but if this means existing guests break, then many people won't be able > to use this option: having to find out which kernel version your guest > is running is a significant burden. > > So on the host side, we need to detect guests that > don't program the IOMMU and make QEMU ignore it. > I think we need to figure out a way to do this > before we commit to the guest change. What is the usecase for putting virtio devices behind an IOMMU, apart from: 1) "because you can" 2) using VFIO within the guest ? Case 1 can be ignored, and in case 2 the guest will do the right thing. > Additionally, IOMMU overhead is very high when running within the VM. > So for uses such as VFIO, we'd like a way to make something like > iommu-pt the default. That's not something that the kernel cares about. It's just a configuration issue. Paolo