[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]
Subject: Re: [virtio-comment] [PATCH 1/3] shared memory: Define shared memory regions
* 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
[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]