On Fri, Feb 21, 2020 at 05:41:51PM +0100, Christoph Hellwig wrote:
> On Thu, Feb 20, 2020 at 04:33:35PM -0500, Michael S. Tsirkin wrote:
> > So it sounds like a host issue: the emulation of s390 unnecessarily complicated.
> > Working around it by the guest looks wrong ...
> 
> Yes.  If your host (and I don't care if you split hypervisor,
> ultravisor and megavisor out in your implementation) wants to
> support a VM architecture where the host can't access all guest
> memory you need to ensure the DMA API is used.  Extra points for
> simply always setting the flag and thus future proofing the scheme.

Moving towards F_ACCESS_PLATFORM everywhere is a good idea (for other
reasons), but that doesn't make the problem as it exists right now go
away.

But, "you need to ensure the DMA API is used" makes no sense from the
host point of view.  The existence of the DMA API is an entirely guest
side, and Linux specific detail, the host can't make decisions based
on that.

For POWER - possibly s390 as well - the hypervisor has no way of
knowing at machine construction time whether it will be an old kernel
(or non Linux OS) which can't support F_ACCESS_PLATFORM, or a guest
which will enter secure mode and therefore requires F_ACCESS_PLATFORM
(according to you).  That's the fundamental problem here.

The normal virtio model of features that the guest can optionally
accept would work nicely here - except that that wouldn't work for the
case of hardware virtio devices, where the access limitations come
from "host" (platform) side and therefore can't be disabled by that
host.

We really do have two cases here: 1) access restrictions originating
with the host/platform (e.g. hardware virtio) and 2) access
restrictions originating with the guest (e.g. secure VMs).  What we
need to do to deal with them is basically the same at the driver
level, but it has subtle and important differences at the platform
level.

-- 
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