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. > > Turning iommu_platform=on for virtio-ccw makes no sense whatsover, > because for CCW I/O there is no such thing as IOMMU and the addresses > are always physical addresses. Fix the name then. The spec calls is ACCESS_PLATFORM now, which makes much more sense. > > > > > that isn't extensibe for cases that for example require bounce buffering. > > > > In fact bounce buffering isn't really the issue from the hypervisor > > (or spec's) point of view. It's the fact that not all of guest memory > > is accessible to the hypervisor. Bounce buffering is just one way the > > guest might deal with that. > > > > Agreed. > > Regards, > Halil > > >
