On Tue, Jan 22, 2013 at 01:24:35PM -0800, Dave Hansen wrote: > > 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: Dave Hansen <d...@linux.vnet.ibm.com> > Acked-by: Rik van Riel <r...@redhat.com> > --- > > linux-2.6.git-dave/arch/x86/kernel/kvm.c | 9 +++++---- > linux-2.6.git-dave/arch/x86/kernel/kvmclock.c | 4 ++-- > 2 files changed, 7 insertions(+), 6 deletions(-)
Reviewed-by: Marcelo Tosatti <mtosa...@redhat.com> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
