Re: Mapping memory regions on s390

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* Cornelia Huck (cohuck@xxxxxxxxxx) wrote:
> On Mon, 18 Feb 2019 15:41:07 +0000
> "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> wrote:
> 
> > * Cornelia Huck (cohuck@xxxxxxxxxx) wrote:
> 
> > > > >> We can of course switch the order of mappings
> > > > >>
> > > > >> [0x000000000000000      ]
> > > > >> ... Memory region containing RAM
> > > > >> [ram_size         	]
> > > > >> ... Memory region for memory devices (virtio-pmem, virtio-mem ...)
> > > > >> [maxram_size - ram_size ]
> > > > >> ... Memory region for e.g. special PCI/CCW devices
> > > > >> [                    TBD]
> > > > >>
> > > > >> We can size TBD in a way that we e.g. max out the current page table
> > > > >> size before having to switch to more levels.    
> > > > > 
> > > > > Yes, that's fine to set some upper limit; you've just got to make sure
> > > > > that the hypervisor knows where it can put stuff and if the guest
> > > > > does PCI that it knows where it's allowed to put stuff and as long
> > > > > as the two don't overlap everyone is happy.  
> > > 
> > > Hm... is that an issue for pci? Do we need to care, as s390 uses
> > > special instructions anyway? Or do we want to avoid going through them,
> > > so that the guest can use normal read/write?  
> > 
> > That depends.
> > The stuff we use for virtio-fs we need the shared region to be
> > accessible by the guest via normal instructions because we're using for
> > DAX.  For PCI you might be able to avoid it for most other PCI cases.
> 
> So,
> - virtio-fs regions need to be accessible like normal memory, so they
>   need to show up in the region labeled 'TBD' above 

Yes.

>  (it would fine to
>   communicate the 'where' through pci structures)

Hmm, mixing PCI structures into something you're not treating as PCI
seems weird to me.

> - pci uses special instructions on s390, so we probably don't need to
>   care about clashes?

I guess so; as long as they're really different address spaces.

Dave

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



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