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

[Halil Pasic]

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.

> 
> > 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

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