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

From: David Hildenbrand
Subject: Re: [PATCH RFCv3 6/9] s390x/diag: subcode to query device memory region
Date: Mon, 27 Jul 2020 11:52:30 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

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?

> (...)
>> 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.


David / dhildenb

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