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Tsirkin" , Heiko Carstens , qemu-devel@nongnu.org, Halil Pasic , Christian Borntraeger , qemu-s390x@nongnu.org, Claudio Imbrenda , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, 27 Jul 2020 11:52:30 +0200 David Hildenbrand wrote: > On 27.07.20 11:48, Cornelia Huck wrote: > > On Fri, 24 Jul 2020 16:37:47 +0200 > > David Hildenbrand 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? So maybe under doc/specs/?