Re: Mapping memory regions on s390

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On Thu, 21 Feb 2019 13:39:46 +0000
"Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> wrote:

> * 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.

I agree, it does seem weird. Adding Sebastian to CC for his zPCI expertise.
Unfortunately I'm not knowledgeable enough about zPCI. I will try to talk to
him next week (he won't be available until next week).

Regards,
Halil

> 
> > - 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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