Re: [virtio-comment] [PATCH 1/3] shared memory: Define shared memory regions

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Cornelia Huck (cohuck@redhat.com) wrote:
> On Fri, 15 Feb 2019 13:33:06 +0100
> David Hildenbrand <david@redhat.com> wrote:
> 
> > On 15.02.19 13:28, Cornelia Huck wrote:
> > > On Fri, 15 Feb 2019 12:26:00 +0100
> > > David Hildenbrand <david@redhat.com> wrote:
> > >   
> > >> Probing is always ugly. But I think we can add something like
> > >>  the x86 PCI hole between 3 and 4 GB after our initial boot memory.
> > >> So there, we would have a memory region just like e.g. x86 has.  
> > > 
> > > A special region is probably the best way out of this pickle. We would
> > > only need the discovery ccw for virtio, then.
> > >   
> > >>
> > >> This should even work with other mechanism I am working on. E.g.
> > >> for memory devices, we will add yet another memory region above
> > >> the special PCI region.
> > >>
> > >> The layout of the guest would then be something like
> > >>
> > >> [0x000000000000000]
> > >> ... Memory region containing RAM
> > >> [ram_size         ]
> > >> ... Memory region for e.g. special PCI devices
> > >> [ram_size +1 GB   ]
> > >> ... Memory region for memory devices (virtio-pmem, virtio-mem ...)
> > >> [maxram_size - ram_size + 1GB]
> > >>
> > >> We would have to create proper page tables for guest backing that take
> > >> care of the new guest size (not just ram_size). Also, to the guest we
> > >> would indicate "maximum ram size == ram_size" so it does not try to
> > >> probe the "special" memory.  
> > > 
> > > Hm... so that would be:
> > > - 0..ram_size: just like it is handled now
> > > - ram_size..ram_size + 1GB: guest does not treat it as ram, but does
> > >   build page tables for it
> > > - ram_size + 1GB..maxram_size: for whatever memory devices do with it
> > > 
> > > How does the guest probe this? (SCLP?) Or does the guest simply know
> > > via some kind of probable feature that there's a 1GB region there?  
> > 
> > As the guest only "knowns" ram, there is a "maximum ram size" specified
> > via SCLP. An unmodified guest will not probe beyond that.
> 
> Nod.
> 
> > The parts of the 1GB used by a device should be communicated via the
> > paravirtualized device I guess. PCI bars don't really fit I assume, so
> > we might need some virtio-ccw thingy (you're the expert :)) on top. That
> > is one part to be clarified.
> > 
> > I guess the guest does not need to know about the whole 1GB, only per
> > device about the used part. We can then built page tables in the guest
> > for that part when plugging.
> 
> Hm. With my proposal, the guest would get a list of region addresses
> from the device via a new ccw. It could then proceed to set up page
> tables for it and start to use it. As long as it is aware that the
> addresses it will get are beyond max_ram, that should be fine, I think.

Which is the same as my virtio-mmio proposal; the host gets to put it
where ever it sees fit (outside ram) and you've just got a way of
telling the guest where it lives.

Davidh's 1GB window is pretty much how older PCs worked I think;
the problem is that 1GB is never enough and you still need a way
to enumarate what devices are where, so it doesn't help you.
(Our current virtio-fs dax mappings we're using are a few GB).

Dave

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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