• David Gibson's avatar
    kvmppc: Implement H_LOGICAL_CI_{LOAD,STORE} in KVM · 99342cf8
    David Gibson authored
    On POWER, storage caching is usually configured via the MMU - attributes
    such as cache-inhibited are stored in the TLB and the hashed page table.
    
    This makes correctly performing cache inhibited IO accesses awkward when
    the MMU is turned off (real mode).  Some CPU models provide special
    registers to control the cache attributes of real mode load and stores but
    this is not at all consistent.  This is a problem in particular for SLOF,
    the firmware used on KVM guests, which runs entirely in real mode, but
    which needs to do IO to load the kernel.
    
    To simplify this qemu implements two special hypercalls, H_LOGICAL_CI_LOAD
    and H_LOGICAL_CI_STORE which simulate a cache-inhibited load or store to
    a logical address (aka guest physical address).  SLOF uses these for IO.
    
    However, because these are implemented within qemu, not the host kernel,
    these bypass any IO devices emulated within KVM itself.  The simplest way
    to see this problem is to attempt to boot a KVM guest from a virtio-blk
    device with iothread / dataplane enabled.  The iothread code relies on an
    in kernel implementation of the virtio queue notification, which is not
    triggered by the IO hcalls, and so the guest will stall in SLOF unable to
    load the guest OS.
    
    This patch addresses this by providing in-kernel implementations of the
    2 hypercalls, which correctly scan the KVM IO bus.  Any access to an
    address not handled by the KVM IO bus will cause a VM exit, hitting the
    qemu implementation as before.
    
    Note that a userspace change is also required, in order to enable these
    new hcall implementations with KVM_CAP_PPC_ENABLE_HCALL.
    Signed-off-by: default avatarDavid Gibson <david@gibson.dropbear.id.au>
    [agraf: fix compilation]
    Signed-off-by: default avatarAlexander Graf <agraf@suse.de>
    99342cf8
book3s_pr_papr.c 10.2 KB