qemu currently implements the hypercalls H_LOGICAL_CI_LOAD and
H_LOGICAL_CI_STORE as PAPR extensions. These are used by the SLOF firmware
for IO, because performing cache inhibited MMIO accesses with the MMU off
(real mode) is very awkward on POWER.
This approach breaks when SLOF needs to access IO devices implemented
within KVM instead of in qemu. The simplest example would be virtio-blk
using an iothread, because the iothread / dataplane mechanism relies on
an in-kernel implementation of the virtio queue notification MMIO.
To fix this, an in-kernel implementation of these hypercalls has been made,
(kernel commit 99342cf "kvmppc: Implement H_LOGICAL_CI_{LOAD,STORE} in KVM"
however, the hypercalls still need to be enabled from qemu. This performs
the necessary calls to do so.
It would be nice to provide some warning if we encounter a problematic
device with a kernel which doesn't support the new calls. Unfortunately,
I can't see a way to detect this case which won't either warn in far too
many cases that will probably work, or which is horribly invasive.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
026bfd89cb