* Cornelia Huck (cohuck@xxxxxxxxxx) wrote: > On Thu, 21 Feb 2019 15:11:50 +0000 > "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> wrote: > > > * Cornelia Huck (cohuck@xxxxxxxxxx) wrote: > > > 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 was thinking about the addresses in the TBD area... they need to go > > > through _some_ pci structure, I assume? > > > > Well I think it depends how you make it work with CCW; > > > > if the addresses being assigned are assigned by the host then I believe you should use > > the CCW mechanism you suggested to discover the addresses in the guest. > > > > If the host is going to allocate a block of PCI space and the guest is > > going to allocate the use within that space and access it with normal > > instructions then I think it should go via PCI. > > This is getting a bit confusing... let me try to summarize: > > - We introduce a special area where shared memory areas are supposed to > live. > - If a virtio device accessed via ccw defines shared regions, the > driver can discover them via a new ccw that indicates an address in > that special area. Right. > - If a virtio device accessed via pci defines shared regions, the > driver will want to discover them via the same mechanism as on other > platforms. If I read > https://lists.oasis-open.org/archives/virtio-comment/201901/msg00003.html > correctly, this will mean an offset into a BAR. This will be a normal > pci memory region. Right; note that like any other BAR it's upto the guest to write the BAR to select which area of GPA it wants the BAR to map to. > Now, this sounds to me that we'll have regions in different memory > regions, depending on whether they are accessed via ccw or via pci. Not > sure if that's a problem. As long as they stay out of each others way it shouldn't be. Dave -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
