From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC 7/11] virtio_pci: new, capability-aware driver. Date: Wed, 11 Jan 2012 17:39:55 +0200 Message-ID: <20120111153954.GC20570@redhat.com> References: <87bor5nlht.fsf@rustcorp.com.au> <20111219091324.GA19535@redhat.com> <871us0om2t.fsf@rustcorp.com.au> <20111220113718.GF3913@redhat.com> <878vm6daqy.fsf@rustcorp.com.au> <20120110170334.GA18404@redhat.com> <8762gj6q5r.fsf@rustcorp.com.au> <1326273019.23910.118.camel@pasglop> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: 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: Stefan Hajnoczi Cc: Pawel Moll , Benjamin Herrenschmidt , virtualization , Christian Borntraeger , Sasha Levin List-Id: virtualization@lists.linuxfoundation.org On Wed, Jan 11, 2012 at 02:28:48PM +0000, Stefan Hajnoczi wrote: > On Wed, Jan 11, 2012 at 9:10 AM, Benjamin Herrenschmidt > wrote: > > On Wed, 2012-01-11 at 08:47 +0000, Stefan Hajnoczi wrote: > >> > >> This is also an opportunity to stop using CPU physical addresses in > >> the ring and instead perform DMA like a normal PCI device (use bus > >> addresses). > > > > Euh why ? > > Because it's a paravirt hack that ends up hitting corner cases. It's > not possible to do virtio-pci passthrough under nested virtualization > unless we use an IOMMU. Imagine passing virtio-net from L0 into the > L2 guest (i.e. PCI-passthrough). If virtio-pci is really "PCI" this > should be possible but it's not when we use physical addresses instead > of bus addresses. > > Stefan It won't be hard to show siginificant performance regression if we do this. Hard to justify for something as niche as nested virt. -- MST