[Moving this away from virtio-comments, as requested; instead cc:ing linux-s390, where there's a chance of s390 folks commenting. For those new to the thread, here's some background: We want to be able to map shared memory regions from the host into the guest for virtio devices, see https://lists.oasis-open.org/archives/virtio-comment/201901/msg00000.html . This needs to be modeled for virtio-ccw on s390 as well, see https://lists.oasis-open.org/archives/virtio-comment/201902/msg00002.html and following. The question is how to map host regions into the guest so that they don't get into the way of normal operation. David made a proposal with a 1 GB memory hole which is similar to that seen on older PCs.] On Fri, 15 Feb 2019 23:08:12 +0100 David Hildenbrand <david@xxxxxxxxxx> wrote: > >> On s390x, we have to define the size of the host->guest page table when > >> starting the guest. So we need some upper limit. > > > > That's OK; x86 also has that because they have a limited physical > > and virtual address size [which may or may not be correctly passed to > > the guest!]. > > > >> Mapping anywhere, I > >> really don't like. Letting the guest define the mapping, I really don't > >> like. > > > > Well it's OK to have a hole for it, but letting the guest choose where > > those mappings go in the hole is the norm for PCI (there are > > exceptions). > > Yes, but at least if we need a new interface for this for s390x (what I > assume), not allowing the guest to map is obviously easier. However if > there is good reason why it is needed, nothing speaks against it. We > just have to offer the guest enough information to do it. Speaking of host/guest: It's probably fine to do something that requires a hypervisor; it just needs to work within the constraints of the architecture. Not sure if SCLP or a diag hypercall should come into play. > > > > >> 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? > > > > [We should probably take this level of detail off this list - it's > > parsecs away from the detail of virtio] > > Yes indeed, I guess this gives Conny an idea on how it could work, so > now she can design an interface - and clarify all the details ;) I really need someone familiar with memory handling for that, though :) > I guess we'll need something for both, virtio-ccw and virtio-pci on s390x. My guess is that we can use the same mechanism for pci. The hard part is probably coming up with a memory layout...
