Re: [PATCH RFCv3 6/9] s390x/diag: subcode to query device memory region

[David Hildenbrand]

On 27.07.20 12:09, Cornelia Huck wrote:
> On Mon, 27 Jul 2020 11:52:30 +0200
> David Hildenbrand <david@redhat.com> wrote:
> 
>> On 27.07.20 11:48, Cornelia Huck wrote:
>>> On Fri, 24 Jul 2020 16:37:47 +0200
>>> David Hildenbrand <david@redhat.com> wrote:
>>>   
>>>> A guest OS that is aware of memory devices (placed into the device
>>>> memory region located in guest physical address space) has to know at least
>>>> the end address of the device memory region during boot, for example, to
>>>> prepare the kernel virtual address space accordingly (e.g., select page
>>>> table hierarchy). The device memory region is located above the SCLP
>>>> maximum storage increment.
>>>>
>>>> Let's provide a new diag500 subcode to query the location of the device
>>>> memory region under QEMU/KVM. This way, esp. Linux who's wants to support
>>>> virtio-based memory devices can query the location of this region and
>>>> derive the maximum possible PFN.
>>>>
>>>> Let's use a specification exception in case no such memory region
>>>> exists (e.g., maxmem wasn't specified, or on old QEMU machines). We'll
>>>> unlock this with future patches that prepare and instanciate the device
>>>> memory region.  
>>>
>>> Specification exception on old machines seems reasonable. But maybe
>>> newer machines can use a different return value for "no memory regions"?  
>>
>> Hm, I don't see any benefit to distinguish the two cases of "no device
>> memory region". Should the guest really care?
> 
> No idea, was just a random thought.
> 
>>
>> [...]
>>>
>>> (...)
>>>   
>>>> diff --git a/hw/s390x/s390-hypercall.h b/hw/s390x/s390-hypercall.h
>>>> index e6b958db41..1b179d7d99 100644
>>>> --- a/hw/s390x/s390-hypercall.h
>>>> +++ b/hw/s390x/s390-hypercall.h
>>>> @@ -16,6 +16,7 @@
>>>>  #define DIAG500_VIRTIO_RESET           1 /* legacy */
>>>>  #define DIAG500_VIRTIO_SET_STATUS      2 /* legacy */
>>>>  #define DIAG500_VIRTIO_CCW_NOTIFY      3 /* KVM_S390_VIRTIO_CCW_NOTIFY */
>>>> +#define DIAG500_DEVICE_MEMORY_REGION   4  
>>>
>>> Regardless what we end up with, this needs to be specified
>>> somewhere(tm).
>>>   
>>
>> Yeah, there, we should also document the existing subcodes. What would
>> be the right place for this? The kernel feels somewhat wrong to me.
> 
> The still supported subcode 3 is properly specified in the virtio spec.
> That's not a good place for that new one, though.
> 
> QEMU is probably a better place than the kernel to specify stuff,
> although it's not really ideal, either. OTOH, do we ever expect other
> hypervisors to implement this new subcode?

cloud-hypervisor implements virtio-mem. If it were ever to support s390x
(guess it does not yet), it would also want to implement that one. But
then, it can just look at QEMU doc I guess :)


-- 
Thanks,

David / dhildenb