• Dave Hansen's avatar
    x86, kvm: Fix kvm's use of __pa() on percpu areas · 5dfd486c
    Dave Hansen authored
    In short, it is illegal to call __pa() on an address holding
    a percpu variable.  This replaces those __pa() calls with
    slow_virt_to_phys().  All of the cases in this patch are
    in boot time (or CPU hotplug time at worst) code, so the
    slow pagetable walking in slow_virt_to_phys() is not expected
    to have a performance impact.
    
    The times when this actually matters are pretty obscure
    (certain 32-bit NUMA systems), but it _does_ happen.  It is
    important to keep KVM guests working on these systems because
    the real hardware is getting harder and harder to find.
    
    This bug manifested first by me seeing a plain hang at boot
    after this message:
    
    	CPU 0 irqstacks, hard=f3018000 soft=f301a000
    
    or, sometimes, it would actually make it out to the console:
    
    [    0.000000] BUG: unable to handle kernel paging request at ffffffff
    
    I eventually traced it down to the KVM async pagefault code.
    This can be worked around by disabling that code either at
    compile-time, or on the kernel command-line.
    
    The kvm async pagefault code was injecting page faults in
    to the guest which the guest misinterpreted because its
    "reason" was not being properly sent from the host.
    
    The guest passes a physical address of an per-cpu async page
    fault structure via an MSR to the host.  Since __pa() is
    broken on percpu data, the physical address it sent was
    bascially bogus and the host went scribbling on random data.
    The guest never saw the real reason for the page fault (it
    was injected by the host), assumed that the kernel had taken
    a _real_ page fault, and panic()'d.  The behavior varied,
    though, depending on what got corrupted by the bad write.
    Signed-off-by: default avatarDave Hansen <dave@linux.vnet.ibm.com>
    Link: http://lkml.kernel.org/r/20130122212435.4905663F@kernel.stglabs.ibm.comAcked-by: default avatarRik van Riel <riel@redhat.com>
    Reviewed-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
    Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
    5dfd486c
kvmclock.c 7.44 KB