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Subject: Re: [virtio-comment] [PATCH 1/3] shared memory: Define shared memory regions

On 15.02.19 13:37, Cornelia Huck 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.

Sounds like this could work in general.



David / dhildenb

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