Re: [PATCH 1/2] mm: move force_dma_unencrypted() to mem_encrypt.h

[Michael S. Tsirkin]

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
> 
> 
>