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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 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"? > > Memory managed by memory devices should never be detected and used > without having proper support for them in the guest (IOW, a driver that > detects and handles the devices). It's not exposed via other HW/firmware > interfaces (e.g., SCLP, diag260). In the near future, the focus is on > supporting virtio-based memory devices like vitio-mem. Other memory devices > are imaginable in the future (e.g., expose DIMMs via a KVM-specific > interface to s390x guests). > > Note: We don't want to include the device memory region within the > SCLP-defined maximum storage increment, because especially older > guests will will sense (via tprot) accessible memory within this range. > If an unmodified guest would detect and use device memory, it could end > badly. The memory might have different semantics (e.g., a disk provided > via virtio-pmem a.k.a. DAX) and might require a handshake first (e.g., > unplugged memory part of virtio-mem in some cases), before memory that > might look accessible can actually be used without surprises. > > Signed-off-by: David Hildenbrand > --- > hw/s390x/s390-hypercall.c | 18 ++++++++++++++++++ > hw/s390x/s390-hypercall.h | 1 + > 2 files changed, 19 insertions(+) (...) > 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). > > void handle_diag_500(CPUS390XState *env, uintptr_t ra); > #endif /* HW_S390_HYPERCALL_H */