On Tue, Feb 25, 2020 at 07:08:02PM +0100, Cornelia Huck wrote: > On Mon, 24 Feb 2020 19:49:53 +0100 > Halil Pasic <pasic@xxxxxxxxxxxxx> wrote: > > > On Mon, 24 Feb 2020 14:33:14 +1100 > > David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > On Fri, Feb 21, 2020 at 07:07:02PM +0100, Halil Pasic wrote: > > > > On Fri, 21 Feb 2020 10:48:15 -0500 > > > > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > > > > > > > On Fri, Feb 21, 2020 at 02:06:39PM +0100, Halil Pasic wrote: > > > > > > On Fri, 21 Feb 2020 14:27:27 +1100 > > > > > > David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > On Thu, Feb 20, 2020 at 05:31:35PM +0100, Christoph Hellwig wrote: > > > > > > > > On Thu, Feb 20, 2020 at 05:23:20PM +0100, Christian Borntraeger wrote: > > > > > > > > > >From a users perspective it makes absolutely perfect sense to use the > > > > > > > > > bounce buffers when they are NEEDED. > > > > > > > > > Forcing the user to specify iommu_platform just because you need bounce buffers > > > > > > > > > really feels wrong. And obviously we have a severe performance issue > > > > > > > > > because of the indirections. > > > > > > > > > > > > > > > > The point is that the user should not have to specify iommu_platform. > > > > > > > > We need to make sure any new hypervisor (especially one that might require > > > > > > > > bounce buffering) always sets it, > > > > > > > > > > > > > > So, I have draft qemu patches which enable iommu_platform by default. > > > > > > > But that's really because of other problems with !iommu_platform, not > > > > > > > anything to do with bounce buffering or secure VMs. > > > > > > > > > > > > > > The thing is that the hypervisor *doesn't* require bounce buffering. > > > > > > > In the POWER (and maybe s390 as well) models for Secure VMs, it's the > > > > > > > *guest*'s choice to enter secure mode, so the hypervisor has no reason > > > > > > > to know whether the guest needs bounce buffering. As far as the > > > > > > > hypervisor and qemu are concerned that's a guest internal detail, it > > > > > > > just expects to get addresses it can access whether those are GPAs > > > > > > > (iommu_platform=off) or IOVAs (iommu_platform=on). > > > > > > > > > > > > I very much agree! > > > > > > > > > > > > > > > > > > > > > as was a rather bogus legacy hack > > > > > > > > > > > > > > It was certainly a bad idea, but it was a bad idea that went into a > > > > > > > public spec and has been widely deployed for many years. We can't > > > > > > > just pretend it didn't happen and move on. > > > > > > > > > > > > > > Turning iommu_platform=on by default breaks old guests, some of which > > > > > > > we still care about. We can't (automatically) do it only for guests > > > > > > > that need bounce buffering, because the hypervisor doesn't know that > > > > > > > ahead of time. > > We could default to iommu_platform=on on s390 when the host has active > support for protected virtualization... but that's just another kind of > horrible, so let's just pretend I didn't suggest it. Yeah, that would break migration between hosts with the feature and hosts without - for everything, not just protected guests. In general any kind of guest visible configuration change based on host properties is incompatible with the qemu/KVM migration model. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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