On Wed, Jun 02, 2010 at 02:19:28PM +0200, Joerg Roedel wrote: > On Wed, Jun 02, 2010 at 02:21:00PM +0300, Michael S. Tsirkin wrote: > > On Wed, Jun 02, 2010 at 01:12:25PM +0200, Joerg Roedel wrote: > > > > Even if it is bound to a domain the userspace driver could program the > > > device to do dma to unmapped regions causing io-page-faults. The kernel > > > can't do anything about it. > > > > It can always corrupt its own memory directly as well :) > > But that is not a reason not to detect errors if we can, > > and not to make APIs hard to misuse. > > Changing the domain of a device while dma can happen is the same type of > bug as unmapping potential dma target addresses. We can't catch this > kind of misuse. you normally need device mapped to start DMA. SHARE makes this bug more likely as you allow switching domains: mmap could be done before switching. > > > > With 10 devices you have 10 extra ioctls. > > > > > > And this works implicitly with your proposal? > > > > Yes. so you do: > > iommu = open > > ioctl(dev1, BIND, iommu) > > ioctl(dev2, BIND, iommu) > > ioctl(dev3, BIND, iommu) > > ioctl(dev4, BIND, iommu) > > > > No need to add a SHARE ioctl. > > In my proposal this looks like: > > > dev1 = open(); > ioctl(dev2, SHARE, dev1); > ioctl(dev3, SHARE, dev1); > ioctl(dev4, SHARE, dev1); > > So we actually save an ioctl. I thought we had a BIND ioctl? > > > Remember that we still need to be able to provide seperate mappings > > > for each device to support IOMMU emulation for the guest. > > > > Generally not true. E.g. guest can enable iommu passthrough > > or have domain per a group of devices. > > What I meant was that there may me multiple io-addresses spaces > necessary for one process. I didn't want to say that every device > _needs_ to have its own address space. > > > > As I wrote the domain has a reference count and is destroyed only when > > > it goes down to zero. This does not happen as long as a device is bound > > > to it. > > > > > > Joerg > > > > We were talking about UNSHARE ioctl: > > ioctl(dev1, UNSHARE, dev2) > > Does it change the domain for dev1 or dev2? > > If you make a mistake you get a hard to debug bug. > > As I already wrote we would have an UNBIND ioctl which just removes a > device from its current domain. UNBIND is better than UNSHARE for > exactly the reason you pointed out above. I thought I stated that > already. > > Joerg You undo SHARE with UNBIND? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
