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

[Paul Mackerras <paulus@ozlabs.org>]

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.