Re: [PATCH v2] KVM: PPC: Book3S PR: only install valid SLBs during KVM_SET_SREGS

On Mon, Oct 02, 2017 at 10:40:22AM +0200, Greg Kurz wrote:

Userland passes an array of 64 SLB descriptors to KVM_SET_SREGS,
some of which are valid (ie, SLB_ESID_V is set) and the rest are
likely all-zeroes (with QEMU at least).

Each of them is then passed to kvmppc_mmu_book3s_64_slbmte(), which
assumes to find the SLB index in the 3 lower bits of its rb argument.
When passed zeroed arguments, it happily overwrites the 0th SLB entry
with zeroes. This is exactly what happens while doing live migration
with QEMU when the destination pushes the incoming SLB descriptors to
KVM PR. When reloading the SLBs at the next synchronization, QEMU first
clears its SLB array and only restore valid ones, but the 0th one is
now gone and we cannot access the corresponding memory anymore:

(qemu) x/x $pc
c0000000000b742c: Cannot access memory

To avoid this, let's filter out non-valid SLB entries. While here, we
also force a full SLB flush before installing new entries.

With this, a 32-bit powermac config with PR KVM enabled fails to build:

  CC [M]  arch/powerpc/kvm/book3s_pr.o
/home/paulus/kernel/kvm/arch/powerpc/kvm/book3s_pr.c: In function ‘kvm_arch_vcpu_ioctl_set_sregs_pr’:
/home/paulus/kernel/kvm/arch/powerpc/kvm/book3s_pr.c:1337:13: error: ‘SLB_ESID_V’ undeclared (first use in this function)
    if (rb & SLB_ESID_V)
             ^
/home/paulus/kernel/kvm/arch/powerpc/kvm/book3s_pr.c:1337:13: note: each undeclared identifier is reported only once for each function it appears in
/home/paulus/kernel/kvm/scripts/Makefile.build:313: recipe for target 'arch/powerpc/kvm/book3s_pr.o' failed
make[3]: *** [arch/powerpc/kvm/book3s_pr.o] Error 1

Paul.